Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 38040)
+++ src/wp-includes/functions.php	(working copy)
@@ -5366,6 +5366,9 @@
  * @return bool                True if the object type exists, false if not.
  */
 function wp_object_type_exists( $object_type ) {
+	$object_type = explode( ':', $object_type );
+	$object_type = $object_type[0];
+
 	/**
 	 * Filters WordPress object types.
 	 *
Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 38041)
+++ src/wp-includes/meta.php	(working copy)
@@ -44,7 +44,8 @@
 		return false;
 	}
 
-	$column = sanitize_key($meta_type . '_id');
+	$meta_type_column = explode( ':', $meta_type );
+	$column = sanitize_key( $meta_type_column[0] . '_id' );
 
 	// expected_slashed ($meta_key)
 	$meta_key = wp_unslash($meta_key);
@@ -157,8 +158,9 @@
 		return false;
 	}
 
-	$column = sanitize_key($meta_type . '_id');
-	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
+	$meta_type_column = explode( ':', $meta_type );
+	$column = sanitize_key( $meta_type_column[0] . '_id' );
+	$id_column = 'user' == $meta_type_column[0] ? 'umeta_id' : 'meta_id';
 
 	// expected_slashed ($meta_key)
 	$raw_meta_key = $meta_key;
@@ -321,8 +323,9 @@
 		return false;
 	}
 
-	$type_column = sanitize_key($meta_type . '_id');
-	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
+	$meta_type_column = explode( ':', $meta_type );
+	$type_column = sanitize_key( $meta_type_column[0] . '_id' );
+	$id_column = 'user' == $meta_type_column[0] ? 'umeta_id' : 'meta_id';
 	// expected_slashed ($meta_key)
 	$meta_key = wp_unslash($meta_key);
 	$meta_value = wp_unslash($meta_value);
@@ -576,12 +579,13 @@
 		return false;
 	}
 
-	$table = _get_meta_table( $meta_type );
+	$meta_type_column = explode( ':', $meta_type );
+	$table = _get_meta_table( $meta_type_column[0] );
 	if ( ! $table ) {
 		return false;
 	}
 
-	$id_column = ( 'user' == $meta_type ) ? 'umeta_id' : 'meta_id';
+	$id_column = ( 'user' == $meta_type_column[0] ) ? 'umeta_id' : 'meta_id';
 
 	$meta = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $table WHERE $id_column = %d", $meta_id ) );
 
@@ -620,13 +624,14 @@
 		return false;
 	}
 
-	$table = _get_meta_table( $meta_type );
+	$meta_type_column = explode( ':', $meta_type );
+	$table = _get_meta_table( $meta_type_column[0] );
 	if ( ! $table ) {
 		return false;
 	}
 
-	$column = sanitize_key($meta_type . '_id');
-	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
+	$column = sanitize_key( $meta_type_column[0] . '_id' );
+	$id_column = 'user' == $meta_type_column[0] ? 'umeta_id' : 'meta_id';
 
 	// Fetch the meta and go on if it's found.
 	if ( $meta = get_metadata_by_mid( $meta_type, $meta_id ) ) {
@@ -711,14 +716,15 @@
 		return false;
 	}
 
-	$table = _get_meta_table( $meta_type );
+	$meta_type_column = explode( ':', $meta_type );
+	$table = _get_meta_table( $meta_type_column[0] );
 	if ( ! $table ) {
 		return false;
 	}
 
 	// object and id columns
-	$column = sanitize_key($meta_type . '_id');
-	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
+	$column = sanitize_key( $meta_type_column[0] . '_id');
+	$id_column = 'user' == $meta_type_column[0] ? 'umeta_id' : 'meta_id';
 
 	// Fetch the meta and go on if it's found.
 	if ( $meta = get_metadata_by_mid( $meta_type, $meta_id ) ) {
@@ -792,12 +798,13 @@
 		return false;
 	}
 
-	$table = _get_meta_table( $meta_type );
+	$meta_type_column = explode( ':', $meta_type );
+	$table = _get_meta_table( $meta_type_column[0] );
 	if ( ! $table ) {
 		return false;
 	}
 
-	$column = sanitize_key($meta_type . '_id');
+	$column = sanitize_key( $meta_type_column[0] . '_id' );
 
 	if ( !is_array($object_ids) ) {
 		$object_ids = preg_replace('|[^0-9,]|', '', $object_ids);
@@ -806,7 +813,7 @@
 
 	$object_ids = array_map('intval', $object_ids);
 
-	$cache_key = $meta_type . '_meta';
+	$cache_key = $meta_type_column[0] . '_meta';
 	$ids = array();
 	$cache = array();
 	foreach ( $object_ids as $id ) {
@@ -822,7 +829,7 @@
 
 	// Get meta info
 	$id_list = join( ',', $ids );
-	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
+	$id_column = 'user' == $meta_type_column[0] ? 'umeta_id' : 'meta_id';
 	$meta_list = $wpdb->get_results( "SELECT $column, meta_key, meta_value FROM $table WHERE $column IN ($id_list) ORDER BY $id_column ASC", ARRAY_A );
 
 	if ( !empty($meta_list) ) {
@@ -883,8 +890,9 @@
  * @return array Associative array of `JOIN` and `WHERE` SQL.
  */
 function get_meta_sql( $meta_query, $type, $primary_table, $primary_id_column, $context = null ) {
+	$type = explode( ':', $type );
 	$meta_query_obj = new WP_Meta_Query( $meta_query );
-	return $meta_query_obj->get_sql( $type, $primary_table, $primary_id_column, $context );
+	return $meta_query_obj->get_sql( $type[0], $primary_table, $primary_id_column, $context );
 }
 
 /**
@@ -900,7 +908,8 @@
 function _get_meta_table($type) {
 	global $wpdb;
 
-	$table_name = $type . 'meta';
+	$type = explode( ':', $type );
+	$table_name = $type[0] . 'meta';
 
 	if ( empty($wpdb->$table_name) )
 		return false;
@@ -936,34 +945,14 @@
  * Sanitize meta value.
  *
  * @since 3.1.3
- * @since 4.6.0 Added the `$object_subtype` parameter.
  *
  * @param string $meta_key       Meta key.
  * @param mixed  $meta_value     Meta value to sanitize.
  * @param string $object_type    Type of object the meta is registered to.
- * @param string $object_subtype Optional. Subtype of object. Will inherit the object type by default.
  *
  * @return mixed Sanitized $meta_value.
  */
-function sanitize_meta( $meta_key, $meta_value, $object_type, $object_subtype = '' ) {
-	if ( ! empty( $object_subtype ) ) {
-		/**
-		 * Filters the sanitization of a specific meta key of a specific meta type and subtype.
-		 *
-		 * The dynamic portions of the hook name, `$meta_type`, `$meta_subtype`,
-		 * and `$meta_key`, refer to the metadata object type (comment, post, or user)
-		 * the object subtype, and the meta key value, respectively.
-		 *
-		 * @since 4.6.0
-		 *
-		 * @param mixed  $meta_value      Meta value to sanitize.
-		 * @param string $meta_key        Meta key.
-		 * @param string $object_type     Object type.
-		 * @param string $object_subtype  Object subtype.
-		 */
-		$meta_value = apply_filters( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $meta_value, $meta_key, $object_type, $object_subtype );
-	}
-
+function sanitize_meta( $meta_key, $meta_value, $object_type ) {
 	/**
 	 * Filters the sanitization of a specific meta key of a specific meta type.
 	 *
@@ -972,14 +961,12 @@
 	 * key value, respectively.
 	 *
 	 * @since 3.3.0
-	 * @since 4.6.0 Added the `$object_subtype` parameter.
 	 *
 	 * @param mixed  $meta_value      Meta value to sanitize.
 	 * @param string $meta_key        Meta key.
 	 * @param string $object_type     Object type.
-	 * @param string $object_subtype  Object subtype.
 	 */
-	return apply_filters( "sanitize_{$object_type}_meta_{$meta_key}", $meta_value, $meta_key, $object_type, $object_subtype );
+	return apply_filters( "sanitize_{$object_type}_meta_{$meta_key}", $meta_value, $meta_key, $object_type );
 }
 
 /**
@@ -995,7 +982,6 @@
  * @param array  $args {
  *     Data used to describe the meta key when registered.
  *
- *     @type string $object_subtype    A subtype; e.g. if the object type is "post", the post type.
  *     @type string $type              The type of data associated with this meta key.
  *     @type string $description       A description of the data attached to this meta key.
  *     @type bool   $single            Whether the meta key has one value per object, or an array of values per object.
@@ -1007,9 +993,9 @@
  *
  * @return bool|WP_error True if the meta key was successfully registered in the global array, WP_Error if not.
  *                       Registering a meta key with distinct sanitize and auth callbacks will fire those
- *                       callbacks, but will not add to the global registry as it requires a subtype.
+ *                       callbacks, but will not add to the global registry as it requires CURIE syntax.
  */
-function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
+function register_meta( $object_type, $meta_key, $args = array(), $deprecated = null ) {
 	global $wp_meta_keys;
 
 	if ( ! is_array( $wp_meta_keys ) ) {
@@ -1021,7 +1007,6 @@
 	}
 
 	$defaults = array(
-		'object_subtype'    => '',
 		'type'              => 'string',
 		'description'       => '',
 		'single'            => false,
@@ -1063,14 +1048,24 @@
 	 */
 	$args = apply_filters( 'register_meta_args', $args, $defaults, $object_type, $meta_key );
 
-	// Object subtype is required if using the args style of registration
-	if ( ! $has_old_sanitize_cb && ! $has_old_auth_cb && empty( $args['object_subtype'] ) ) {
-		return new WP_Error( 'register_meta_failed', __( 'Meta must be registered against an object subtype.' ) );
+	// Check for a valid (WP) CURIE string.
+	$object_type = trim( $object_type, ':' );
+
+	// New object type style cannot be used with old function signature.
+	if ( strpos( $object_type, ':' ) && ( $has_old_auth_cb || $has_old_sanitize_cb ) ) {
+		// error.
+		return false;
 	}
 
-	// If `auth_callback` is not provided, fall back to `is_protected_meta()`.
+	/**
+	 * If `auth_callback` is not provided, fall back to `is_protected_meta()`.
+	 *
+	 * To maintain back-compat, this should be processed with only the base object
+	 * type and not CURIE syntax.
+	 */
 	if ( empty( $args['auth_callback'] ) ) {
-		if ( is_protected_meta( $meta_key, $object_type ) ) {
+		$object_type_protected = explode( ':', $object_type );
+		if ( is_protected_meta( $meta_key, $object_type_protected[0] ) ) {
 			$args['auth_callback'] = '__return_false';
 		} else {
 			$args['auth_callback'] = '__return_true';
@@ -1077,35 +1072,27 @@
 		}
 	}
 
-	$object_subtype = '';
-
-	if ( ! empty( $args['object_subtype'] ) ) {
-		$object_subtype = $args['object_subtype'];
+	// Provide a fallback for new object type styles if using new function signature.
+	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb && ! strpos( $object_type, ':' ) ) {
+		if ( 'term' === $object_type ) {
+			$object_type = 'term:category';
+		} else {
+			$object_type = $object_type . ':' . $object_type;
+		}
 	}
 
-	// Back-compat: old sanitize and auth callbacks are applied to all of an object type.
-	if ( $has_old_sanitize_cb && is_callable( $args['sanitize_callback'] ) ) {
-		add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+	if ( is_callable( $args['sanitize_callback'] ) ) {
+		add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 3 );
 	}
 
-	if ( $has_old_auth_cb && is_callable( $args['auth_callback'] ) ) {
+	if ( is_callable( $args['auth_callback'] ) ) {
 		add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
 	}
 
-	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb) {
-		if ( is_callable( $args['sanitize_callback'] ) ) {
-			add_filter( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
-		}
+	// Global registry only contains meta keys registered with CURIE syntax.
+	if ( strpos( $object_type, ':' ) ) {
+		$wp_meta_keys[ $object_type ][ $meta_key ] = $args;
 
-		if ( is_callable( $args['auth_callback'] ) ) {
-			add_filter( "auth_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
-		}
-	}
-
-	// Global registry only contains meta keys registered in the new way with a subtype.
-	if ( ! empty( $object_subtype ) ) {
-		$wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] = $args;
-
 		return true;
 	}
 
@@ -1118,13 +1105,12 @@
  * @since 4.6.0
  *
  * @param string $object_type    The type of object.
- * @param string $object_subtype The subtype of the object type.
  * @param string $meta_key       The meta key.
  *
- * @return bool|WP_error True if the meta key is registered to the object type and subtype. False if not.
+ * @return bool|WP_error True if the meta key is registered to the object type. False if not.
  *                       WP_Error if an invalid object type is passed.
  */
-function registered_meta_key_exists( $object_type, $object_subtype, $meta_key ) {
+function registered_meta_key_exists( $object_type, $meta_key ) {
 	global $wp_meta_keys;
 
 	if ( ! is_array( $wp_meta_keys ) ) {
@@ -1140,11 +1126,7 @@
 		return false;
 	}
 
-	if ( ! isset( $wp_meta_keys[ $object_type ][ $object_subtype ] ) ) {
-		return false;
-	}
-
-	if ( isset( $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] ) ) {
+	if ( isset( $wp_meta_keys[ $object_type ][ $meta_key ] ) ) {
 		return true;
 	}
 
@@ -1157,35 +1139,30 @@
  * @since 4.6.0
  *
  * @param string $object_type    The type of object.
- * @param string $object_subtype The subtype of the object type.
  * @param string $meta_key       The meta key.
  *
  * @return bool|WP_Error True if successful. WP_Error if the meta key is invalid.
  */
-function unregister_meta_key( $object_type, $object_subtype, $meta_key ) {
+function unregister_meta_key( $object_type, $meta_key ) {
 	global $wp_meta_keys;
 
-	if ( ! registered_meta_key_exists( $object_type, $object_subtype, $meta_key ) ) {
+	if ( ! registered_meta_key_exists( $object_type, $meta_key ) ) {
 		return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key' ) );
 	}
 
-	$args = $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ];
+	$args = $wp_meta_keys[ $object_type ][ $meta_key ];
 
 	if ( isset( $args['sanitize_callback'] ) && is_callable( $args['sanitize_callback'] ) ) {
-		remove_filter( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['sanitize_callback'] );
+		remove_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'] );
 	}
 
 	if ( isset( $args['auth_callback'] ) && is_callable( $args['auth_callback'] ) ) {
-		remove_filter( "auth_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['auth_callback'] );
+		remove_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'] );
 	}
 
-	unset( $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] );
+	unset( $wp_meta_keys[ $object_type ][ $meta_key ] );
 
 	// Do some clean up
-	if ( empty( $wp_meta_keys[ $object_type ][ $object_subtype ] ) ) {
-		unset( $wp_meta_keys[ $object_type ][ $object_subtype ] );
-	}
-
 	if ( empty( $wp_meta_keys[ $object_type ] ) ) {
 		unset( $wp_meta_keys[ $object_type ] );
 	}
@@ -1194,16 +1171,15 @@
 }
 
 /**
- * Retrieves a list of registered meta keys for an object type and optionally subtype.
+ * Retrieves a list of registered meta keys for an object type.
  *
  * @since 4.6.0
  *
- * @param string $object_type    The type of object. Post, comment, user, term.
- * @param string $object_subtype Optional. A subtype of the object (e.g. custom post type).
+ * @param string $object_type The type of object. Post, comment, user, term.
  *
  * @return array List of registered meta keys.
  */
-function get_registered_meta_keys( $object_type, $object_subtype = '' ) {
+function get_registered_meta_keys( $object_type ) {
 	global $wp_meta_keys;
 
 	if ( ! isset( $wp_meta_keys[ $object_type ] ) ) {
@@ -1210,15 +1186,7 @@
 		return array();
 	}
 
-	if ( empty( $object_subtype ) && isset( $wp_meta_keys[ $object_type ] ) ) {
-		return $wp_meta_keys[ $object_type ];
-	}
-
-	if ( ! isset( $wp_meta_keys[ $object_type ][ $object_subtype ] ) ) {
-		return array();
-	}
-
-	return $wp_meta_keys[ $object_type ][ $object_subtype ];
+	return $wp_meta_keys[ $object_type ];
 }
 
 /**
@@ -1227,7 +1195,6 @@
  * @since 4.6.0
  *
  * @param string $object_type    Type of object to request metadata for. (e.g. comment, post, term, user)
- * @param string $object_subtype The subtype of the object's type to request metadata for. (e.g. custom post type)
  * @param int    $object_id      ID of the object the metadata is for.
  * @param string $meta_key       Optional. Registered metadata key. If not specified, retrieve all registered
  *                               metadata for the specified object.
@@ -1234,7 +1201,7 @@
  *
  * @return mixed|WP_Error
  */
-function get_registered_metadata( $object_type, $object_subtype, $object_id, $meta_key = '' ) {
+function get_registered_metadata( $object_type, $object_id, $meta_key = '' ) {
 	global $wp_meta_keys;
 
 	if ( ! is_array( $wp_meta_keys ) ) {
@@ -1246,10 +1213,10 @@
 	}
 
 	if ( ! empty( $meta_key ) ) {
-		if ( ! registered_meta_key_exists( $object_type, $object_subtype, $meta_key ) ) {
+		if ( ! registered_meta_key_exists( $object_type, $meta_key ) ) {
 			return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key. Not registered.' ) );
 		}
-		$meta_keys = get_registered_meta_keys( $object_type, $object_subtype );
+		$meta_keys = get_registered_meta_keys( $object_type );
 		$meta_key_data = $meta_keys[ $meta_key ];
 
 		$data = get_metadata( $object_type, $object_id, $meta_key, $meta_key_data['single'] );
@@ -1259,7 +1226,7 @@
 
 	$data = get_metadata( $object_type, $object_id );
 
-	$meta_keys = get_registered_meta_keys( $object_type, $object_subtype );
+	$meta_keys = get_registered_meta_keys( $object_type );
 	$registered_data = array();
 
 	// Someday, array_filter()
Index: tests/phpunit/tests/meta/registerMeta.php
===================================================================
--- tests/phpunit/tests/meta/registerMeta.php	(revision 38041)
+++ tests/phpunit/tests/meta/registerMeta.php	(working copy)
@@ -28,7 +28,7 @@
 		return $meta_key . ' old sanitized';
 	}
 
-	public function _new_sanitize_meta_cb( $meta_value, $meta_key, $object_type, $object_subtype ) {
+	public function _new_sanitize_meta_cb( $meta_value, $meta_key, $object_type ) {
 		return $meta_key . ' new sanitized';
 	}
 
@@ -69,8 +69,8 @@
 	}
 
 	public function test_register_meta_with_valid_object_type_and_object_subtype_returns_true() {
-		$result = register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		$result = register_meta( 'post:post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertTrue( $result );
 	}
@@ -78,22 +78,19 @@
 	public function test_register_meta_with_post_object_type_and_subtype_populates_wp_meta_keys() {
 		global $wp_meta_keys;
 
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'flight_number' );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$expected = array(
-			'post' => array(
-				'post' => array(
-					'flight_number' => array(
-						'object_subtype' => 'post',
-						'type' => 'string',
-						'description' => '',
-						'single' => false,
-						'sanitize_callback' => null,
-						'auth_callback' => '__return_true',
-						'show_in_rest' => false,
-					),
+			'post:post' => array(
+				'flight_number' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -104,22 +101,19 @@
 	public function test_register_meta_with_post_object_type_and_unregistered_subtype_populates_wp_meta_keys() {
 		global $wp_meta_keys;
 
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'not_a_post_type' ) );
+		register_meta( 'post:not_a_post_type', 'flight_number' );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'post', 'not_a_post_type', 'flight_number' );
+		unregister_meta_key( 'post:not_a_post_type', 'flight_number' );
 
 		$expected = array(
-			'post' => array(
-				'not_a_post_type' => array(
-					'flight_number' => array(
-						'object_subtype' => 'not_a_post_type',
-						'type' => 'string',
-						'description' => '',
-						'single' => false,
-						'sanitize_callback' => null,
-						'auth_callback' => '__return_true',
-						'show_in_rest' => false,
-					),
+			'post:not_a_post_type' => array(
+				'flight_number' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -129,22 +123,19 @@
 
 	public function test_register_meta_with_term_object_type_and_category_subtype_populates_wp_meta_keys() {
 		global $wp_meta_keys;
-		register_meta( 'term', 'category_icon', array( 'object_subtype' => 'category' ) );
+		register_meta( 'term:category', 'category_icon' );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'term', 'category', 'category_icon' );
+		unregister_meta_key( 'term:category', 'category_icon' );
 
 		$expected = array(
-			'term' => array(
-				'category' => array(
-					'category_icon' => array(
-						'object_subtype' => 'category',
-						'type' => 'string',
-						'description' => '',
-						'single' => false,
-						'sanitize_callback' => null,
-						'auth_callback' => '__return_true',
-						'show_in_rest' => false,
-					),
+			'term:category' => array(
+				'category_icon' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -154,22 +145,19 @@
 
 	public function test_register_meta_with_comment_object_type_and_subtype_populates_wp_meta_keys() {
 		global $wp_meta_keys;
-		register_meta( 'comment', 'comment_rating', array( 'object_subtype' => 'comment' ) );
+		register_meta( 'comment:comment', 'comment_rating' );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'comment', 'comment', 'comment_rating' );
+		unregister_meta_key( 'comment:comment', 'comment_rating' );
 
 		$expected = array(
-			'comment' => array(
-				'comment' => array(
-					'comment_rating' => array(
-						'object_subtype' => 'comment',
-						'type' => 'string',
-						'description' => '',
-						'single' => false,
-						'sanitize_callback' => null,
-						'auth_callback' => '__return_true',
-						'show_in_rest' => false,
-					),
+			'comment:comment' => array(
+				'comment_rating' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -200,7 +188,7 @@
 
 	public function test_register_meta_with_deprecated_sanitize_callback_parameter_passes_through_filter() {
 		register_meta( 'post', 'old_sanitized_key', array( $this, '_old_sanitize_meta_cb' ) );
-		$meta = sanitize_meta( 'old_sanitized_key', 'unsanitized', 'post', 'post' );
+		$meta = sanitize_meta( 'old_sanitized_key', 'unsanitized', 'post' );
 
 		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb') );
 		remove_filter( 'auth_post_meta_flight_number', '__return_true');
@@ -210,22 +198,19 @@
 
 	public function test_register_meta_with_current_sanitize_callback_populates_wp_meta_keys() {
 		global $wp_meta_keys;
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		register_meta( 'post:post', 'flight_number', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$expected = array(
-			'post' => array(
-				'post' => array(
-					'flight_number' => array(
-						'object_subtype' => 'post',
-						'type' => 'string',
-						'description' => '',
-						'single' => false,
-						'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ),
-						'auth_callback' => '__return_true',
-						'show_in_rest' => false,
-					),
+			'post:post' => array(
+				'flight_number' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ),
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -233,35 +218,35 @@
 	}
 
 	public function test_register_meta_with_current_sanitize_callback_returns_true() {
-		$result = register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		$result = register_meta( 'post:post', 'flight_number', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertTrue( $result );
 	}
 
 	public function test_register_meta_with_new_sanitize_callback_parameter() {
-		register_meta( 'post', 'new_sanitized_key', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
-		$meta = sanitize_meta( 'new_sanitized_key', 'unsanitized', 'post', 'post' );
+		register_meta( 'post:post', 'new_sanitized_key', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		$meta = sanitize_meta( 'new_sanitized_key', 'unsanitized', 'post:post' );
 
-		unregister_meta_key( 'post', 'post', 'new_sanitized_key' );
+		unregister_meta_key( 'post:post', 'new_sanitized_key' );
 
 		$this->assertEquals( 'new_sanitized_key new sanitized', $meta );
 	}
 
 	public function test_register_meta_unregistered_meta_key_removes_sanitize_filter() {
-		register_meta( 'post', 'new_sanitized_key', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
-		unregister_meta_key( 'post', 'post', 'new_sanitized_key' );
+		register_meta( 'post:post', 'new_sanitized_key', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		unregister_meta_key( 'post:post', 'new_sanitized_key' );
 
-		$has_filter = has_filter( 'sanitize_post_post_meta_new_sanitized_key', array( $this, '_new_sanitize_meta_cb' ) );
+		$has_filter = has_filter( 'sanitize_post:post_meta_new_sanitized_key', array( $this, '_new_sanitize_meta_cb' ) );
 
 		$this->assertFalse( $has_filter );
 	}
 
 	public function test_register_meta_unregistered_meta_key_removes_auth_filter() {
-		register_meta( 'post', 'new_auth_key', array( 'object_subtype' => 'post', 'auth_callback' => array( $this, '_new_auth_meta_cb' ) ) );
-		unregister_meta_key( 'post', 'post', 'new_auth_key' );
+		register_meta( 'post:post', 'new_auth_key', array( 'auth_callback' => array( $this, '_new_auth_meta_cb' ) ) );
+		unregister_meta_key( 'post:post', 'new_auth_key' );
 
-		$has_filter = has_filter( 'auth_post_post_meta_new_auth_key', array( $this, '_new_auth_meta_cb' ) );
+		$has_filter = has_filter( 'auth_post:post_meta_new_auth_key', array( $this, '_new_auth_meta_cb' ) );
 
 		$this->assertFalse( $has_filter );
 	}
@@ -268,24 +253,24 @@
 
 	public function test_unregister_meta_key_clears_key_from_wp_meta_keys() {
 		global $wp_meta_keys;
-		register_meta( 'post', 'registered_key', array( 'object_subtype' => 'post' ) );
-		unregister_meta_key( 'post', 'post', 'registered_key' );
+		register_meta( 'post:post', 'registered_key' );
+		unregister_meta_key( 'post:post', 'registered_key' );
 
 		$this->assertEquals( array(), $wp_meta_keys );
 	}
 
 	public function test_unregister_meta_key_with_invalid_key_returns_wp_error() {
-		$this->assertWPError( unregister_meta_key( 'post', 'post', 'not_a_registered_key' ) );
+		$this->assertWPError( unregister_meta_key( 'post:post', 'not_a_registered_key' ) );
 	}
 
 	public function test_get_registered_meta_keys() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
-		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'registered_key1' );
+		register_meta( 'post:post', 'registered_key2' );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+		$meta_keys = get_registered_meta_keys( 'post:post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
-		unregister_meta_key( 'post', 'post', 'registered_key2' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
+		unregister_meta_key( 'post:post', 'registered_key2' );
 
 		$this->assertArrayHasKey( 'registered_key1', $meta_keys );
 		$this->assertArrayHasKey( 'registered_key2', $meta_keys );
@@ -292,118 +277,118 @@
 	}
 
 	public function test_get_registered_meta_keys_with_subtype_without_registered_keys_is_empty() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
-		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'registered_key1' );
+		register_meta( 'post:post', 'registered_key2' );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'page' );
+		$meta_keys = get_registered_meta_keys( 'post:page' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
-		unregister_meta_key( 'post', 'post', 'registered_key2' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
+		unregister_meta_key( 'post:post', 'registered_key2' );
 
 		$this->assertEmpty( $meta_keys );
 	}
 
 	public function test_get_registered_meta_keys_with_invalid_type_is_empty() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
-		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'registered_key1' );
+		register_meta( 'post:post', 'registered_key2' );
 
 		$meta_keys = get_registered_meta_keys( 'invalid-type' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
-		unregister_meta_key( 'post', 'post', 'registered_key2' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
+		unregister_meta_key( 'post:post', 'registered_key2' );
 
 		$this->assertEmpty( $meta_keys );
 	}
 
-	public function test_get_registered_meta_keys_has_count() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
-		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'page' ) );
+	public function test_get_registered_meta_keys_with_only_base_type_has_0_count() {
+		register_meta( 'post:post', 'registered_key1' );
+		register_meta( 'post:page', 'registered_key2' );
 
 		$meta_keys = get_registered_meta_keys( 'post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
-		unregister_meta_key( 'post', 'page', 'registered_key2' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
+		unregister_meta_key( 'post:page', 'registered_key2' );
 
-		$this->assertCount( 2, $meta_keys );
+		$this->assertCount( 0, $meta_keys );
 	}
 
 	public function test_get_registered_meta_keys_description_arg() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post', 'description' => 'I\'m just a field, take a good look at me' ) );
+		register_meta( 'post:post', 'registered_key1', array( 'description' => 'I\'m just a field, take a good look at me' ) );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+		$meta_keys = get_registered_meta_keys( 'post:post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
 
 		$this->assertEquals( 'I\'m just a field, take a good look at me', $meta_keys['registered_key1']['description'] );
 	}
 
 	public function test_get_registered_meta_keys_invalid_arg() {
-		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post', 'invalid_arg' => 'invalid' ) );
+		register_meta( 'post:post', 'registered_key1', array( 'invalid_arg' => 'invalid' ) );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+		$meta_keys = get_registered_meta_keys( 'post:post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( 'post:post', 'registered_key1' );
 
 		$this->assertArrayNotHasKey( 'invalid_arg', $meta_keys['registered_key1'] );
 	}
 
 	public function test_get_registered_metadata() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'flight_number' );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id );
+		$meta = get_registered_metadata( 'post:post', self::$post_id );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertEquals( 'Oceanic 815', $meta['flight_number'][0] );
 	}
 
 	public function test_get_registered_metadata_by_key() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'flight_number' );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_number' );
+		$meta = get_registered_metadata( 'post:post', self::$post_id, 'flight_number' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertEquals( 'Oceanic 815', $meta[0] );
 	}
 
 	public function test_get_registered_metadata_by_key_single() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'single' => true ) );
+		register_meta( 'post:post', 'flight_number', array( 'single' => true ) );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_number' );
+		$meta = get_registered_metadata( 'post:post', self::$post_id, 'flight_number' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertEquals( 'Oceanic 815', $meta );
 	}
 
 	public function test_get_registered_metadata_by_invalid_key() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'flight_number' );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_pilot' );
+		$meta = get_registered_metadata( 'post:post', self::$post_id, 'flight_pilot' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertWPError( $meta );
 	}
 
 	public function test_get_registered_metadata_invalid_object_type() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post:post', 'flight_number' );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
+		$meta = get_registered_metadata( 'invalid-type:invalid-subtype', self::$post_id );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post:post', 'flight_number' );
 
 		$this->assertWPError( $meta );
 	}
 
 	public function test_get_registered_metadata_invalid() {
-		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
+		$meta = get_registered_metadata( 'invalid-type:invalid-subtype', self::$post_id );
 
 		$this->assertWPError( $meta );
 	}
