Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 38094)
+++ src/wp-includes/functions.php	(working copy)
@@ -5358,31 +5358,6 @@
 }
 
 /**
- * Check if an object type exists. By default, these are `post`, `comment`, `user`, and `term`.
- *
- * @since 4.6.0
- *
- * @param  string $object_type Object type to check.
- * @return bool                True if the object type exists, false if not.
- */
-function wp_object_type_exists( $object_type ) {
-	/**
-	 * Filters WordPress object types.
-	 *
-	 * @since 4.6.0
-	 *
-	 * @param array  $types Array of object types.
-	 */
-	$types = apply_filters( 'wp_object_types', array( 'post', 'comment', 'user', 'term' ) );
-
-	if ( in_array( $object_type, $types ) ) {
-		return true;
-	}
-
-	return false;
-}
-
-/**
  * Attempts to raise the PHP memory limit for memory intensive processes.
  *
  * Only allows raising the existing limit and prevents lowering it.
Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 38094)
+++ src/wp-includes/meta.php	(working copy)
@@ -936,34 +936,14 @@
  * Sanitize meta value.
  *
  * @since 3.1.3
- * @since 4.6.0 Added the `$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, $object_type ) {
 	/**
 	 * Filters the sanitization of a specific meta key of a specific meta type.
 	 *
@@ -972,14 +952,12 @@
 	 * key value, respectively.
 	 *
 	 * @since 3.3.0
-	 * @since 4.6.0 Added the `$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 +973,6 @@
  * @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.
@@ -1005,9 +982,9 @@
  * }
  * @param string|array $deprecated Deprecated. Use `$args` instead.
  *
- * @return bool|WP_error True if the meta key was successfully registered in the global array, WP_Error if not.
+ * @return bool True if the meta key was successfully registered in the global array, false 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.
  */
 function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
 	global $wp_meta_keys;
@@ -1016,12 +993,7 @@
 		$wp_meta_keys = array();
 	}
 
-	if ( ! wp_object_type_exists( $object_type ) ) {
-		return new WP_Error( 'register_meta_failed', __( 'Meta can only be registered against a core object type.' ) );
-	}
-
 	$defaults = array(
-		'object_subtype'    => '',
 		'type'              => 'string',
 		'description'       => '',
 		'single'            => false,
@@ -1049,8 +1021,6 @@
 		$has_old_auth_cb = true;
 	}
 
-	$args = wp_parse_args( $args, $defaults );
-
 	/**
 	 * Filters the registration arguments when registering meta.
 	 *
@@ -1062,12 +1032,8 @@
 	 * @param string $meta_key    Meta key.
 	 */
 	$args = apply_filters( 'register_meta_args', $args, $defaults, $object_type, $meta_key );
+	$args = wp_parse_args( $args, $defaults );
 
-	// 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.' ) );
-	}
-
 	// If `auth_callback` is not provided, fall back to `is_protected_meta()`.
 	if ( empty( $args['auth_callback'] ) ) {
 		if ( is_protected_meta( $meta_key, $object_type ) ) {
@@ -1077,39 +1043,23 @@
 		}
 	}
 
-	$object_subtype = '';
-
-	if ( ! empty( $args['object_subtype'] ) ) {
-		$object_subtype = $args['object_subtype'];
-	}
-
 	// 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 the array of arguments added in 4.6.0.
+	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb ) {
+		$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;
 	}
 
-	return new WP_Error( 'register_meta_failed', __( 'Sanitize and auth callbacks registered; meta key not registered.' ) );
+	return false;
 }
 
 /**
@@ -1118,13 +1068,11 @@
  * @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.
- *                       WP_Error if an invalid object type is passed.
+ * @return bool True if the meta key is registered to the object type. False if not.
  */
-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 ) ) {
@@ -1131,20 +1079,11 @@
 		return false;
 	}
 
-	// Only specific core object types are supported.
-	if ( ! wp_object_type_exists( $object_type ) ) {
-		return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key. Not a core object type.' ) );
-	}
-
 	if ( ! isset( $wp_meta_keys[ $object_type ] ) ) {
 		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 +1096,30 @@
  * @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.
+ * @return bool True if successful. False if the meta key was not registered.
  */
-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 ) ) {
-		return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key' ) );
+	if ( ! registered_meta_key_exists( $object_type, $meta_key ) ) {
+		return false;
 	}
 
-	$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 +1128,15 @@
 }
 
 /**
- * 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 +1143,7 @@
 		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 ];
 }
 
 /**
@@ -1226,30 +1151,20 @@
  *
  * @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.
+ * @param string $object_type Type of object to request metadata for. (e.g. comment, post, term, user)
+ * @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.
  *
- * @return mixed|WP_Error
+ * @return mixed A single value or array of values for a key if specified. An array of all registered keys
+ *               and values for an object ID if not.
  */
-function get_registered_metadata( $object_type, $object_subtype, $object_id, $meta_key = '' ) {
-	global $wp_meta_keys;
-
-	if ( ! is_array( $wp_meta_keys ) ) {
-		return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key. Not registered.' ) );
-	}
-
-	if ( ! wp_object_type_exists( $object_type ) ) {
-		return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key. Not a core object type.' ) );
-	}
-
+function get_registered_metadata( $object_type, $object_id, $meta_key = '' ) {
 	if ( ! empty( $meta_key ) ) {
-		if ( ! registered_meta_key_exists( $object_type, $object_subtype, $meta_key ) ) {
-			return new WP_Error( 'invalid_meta_key', __( 'Invalid meta key. Not registered.' ) );
+		if ( ! registered_meta_key_exists( $object_type, $meta_key ) ) {
+			return false;
 		}
-		$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 +1174,7 @@
 
 	$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 38094)
+++ tests/phpunit/tests/meta/registerMeta.php	(working copy)
@@ -3,32 +3,21 @@
  * @group meta
  */
 class Tests_Meta_Register_Meta extends WP_UnitTestCase {
-	protected static $editor_id;
 	protected static $post_id;
-	protected static $comment_id;
 
 	public static function wpSetUpBeforeClass( $factory ) {
-		self::$editor_id = $factory->user->create( array( 'role' => 'editor' ) );
 		self::$post_id = $factory->post->create();
-		self::$comment_id = $factory->comment->create( array( 'comment_post_ID' => self::$post_id ) );
 	}
 
 	public static function wpTearDownAfterClass() {
-		self::delete_user( self::$editor_id );
-		wp_delete_comment( self::$comment_id, true );
 		wp_delete_post( self::$post_id, true );
 	}
 
-	function setUp() {
-		parent::setUp();
-		wp_set_current_user( self::$editor_id );
-	}
-
 	public function _old_sanitize_meta_cb( $meta_value, $meta_key, $meta_type ) {
 		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';
 	}
 
@@ -68,32 +57,29 @@
 		$this->assertEquals( array( 'auth' => 10, 'sanitize' => 10 ), $has_filters );
 	}
 
-	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' );
+	public function test_register_meta_with_post_object_type_returns_true() {
+		$result = register_meta( 'post', 'flight_number', array() );
+		unregister_meta_key( 'post', 'flight_number' );
 
 		$this->assertTrue( $result );
 	}
 
-	public function test_register_meta_with_post_object_type_and_subtype_populates_wp_meta_keys() {
+	public function test_register_meta_with_post_object_type_populates_wp_meta_keys() {
 		global $wp_meta_keys;
 
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post', 'flight_number', array() );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( '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,
-					),
+				'flight_number' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -101,50 +87,21 @@
 		$this->assertEquals( $actual, $expected );
 	}
 
-	public function test_register_meta_with_post_object_type_and_unregistered_subtype_populates_wp_meta_keys() {
+	public function test_register_meta_with_term_object_type_populates_wp_meta_keys() {
 		global $wp_meta_keys;
-
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'not_a_post_type' ) );
+		register_meta( 'term', 'category_icon', array() );
 		$actual = $wp_meta_keys;
-		unregister_meta_key( 'post', 'not_a_post_type', 'flight_number' );
+		unregister_meta_key( 'term', 'category_icon' );
 
 		$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,
-					),
-				),
-			),
-		);
-
-		$this->assertEquals( $actual, $expected );
-	}
-
-	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' ) );
-		$actual = $wp_meta_keys;
-		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,
-					),
+				'category_icon' => array(
+					'type' => 'string',
+					'description' => '',
+					'single' => false,
+					'sanitize_callback' => null,
+					'auth_callback' => '__return_true',
+					'show_in_rest' => false,
 				),
 			),
 		);
@@ -152,31 +109,6 @@
 		$this->assertEquals( $actual, $expected );
 	}
 
-	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' ) );
-		$actual = $wp_meta_keys;
-		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,
-					),
-				),
-			),
-		);
-
-		$this->assertEquals( $actual, $expected );
-	}
-
 	public function test_register_meta_with_deprecated_sanitize_callback_does_not_populate_wp_meta_keys() {
 		global $wp_meta_keys;
 
@@ -188,14 +120,13 @@
 		$this->assertEquals( array(), $actual );
 	}
 
-	public function test_register_meta_with_deprecated_sanitize_callback_param_returns_wp_error() {
+	public function test_register_meta_with_deprecated_sanitize_callback_param_returns_false() {
 		$actual = register_meta( 'post', 'flight_number', array( $this, '_old_sanitize_meta_cb' ) );
 
 		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb') );
 		remove_filter( 'auth_post_meta_flight_number', '__return_true');
 
-		$this->assertWPError( $actual );
-		$this->assertEquals( 'register_meta_failed', $actual->get_error_code() );
+		$this->assertFalse( $actual );
 	}
 
 	public function test_register_meta_with_deprecated_sanitize_callback_parameter_passes_through_filter() {
@@ -210,22 +141,19 @@
 
 	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', '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', '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,
-					),
+				'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 +161,35 @@
 	}
 
 	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', 'flight_number', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		unregister_meta_key( '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', 'new_sanitized_key', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		$meta = sanitize_meta( 'new_sanitized_key', 'unsanitized', 'post' );
 
-		unregister_meta_key( 'post', 'post', 'new_sanitized_key' );
+		unregister_meta_key( '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', 'new_sanitized_key', array( 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		unregister_meta_key( '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_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', 'new_auth_key', array( 'auth_callback' => array( $this, '_new_auth_meta_cb' ) ) );
+		unregister_meta_key( '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_meta_new_auth_key', array( $this, '_new_auth_meta_cb' ) );
 
 		$this->assertFalse( $has_filter );
 	}
@@ -268,143 +196,108 @@
 
 	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', 'registered_key', array() );
+		unregister_meta_key( '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' ) );
+	public function test_unregister_meta_key_with_invalid_key_returns_false() {
+		$this->assertFalse( unregister_meta_key( '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', 'registered_key1', array() );
+		register_meta( 'post', 'registered_key2', array() );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+		$meta_keys = get_registered_meta_keys( 'post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
-		unregister_meta_key( 'post', 'post', 'registered_key2' );
+		unregister_meta_key( 'post', 'registered_key1' );
+		unregister_meta_key( 'post', 'registered_key2' );
 
 		$this->assertArrayHasKey( 'registered_key1', $meta_keys );
 		$this->assertArrayHasKey( 'registered_key2', $meta_keys );
 	}
 
-	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' ) );
-
-		$meta_keys = get_registered_meta_keys( 'post', 'page' );
-
-		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', 'registered_key1', array() );
+		register_meta( 'post', 'registered_key2', array() );
 
 		$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', 'registered_key1' );
+		unregister_meta_key( '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_description_arg() {
+		register_meta( 'post', 'registered_key1', array( 'description' => 'I\'m just a field, take a good look at me' ) );
 
 		$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', 'registered_key1' );
 
-		$this->assertCount( 2, $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' ) );
-
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
-
-		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', 'registered_key1', array( 'invalid_arg' => 'invalid' ) );
 
-		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+		$meta_keys = get_registered_meta_keys( 'post' );
 
-		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( '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', 'flight_number', array() );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id );
+		$meta = get_registered_metadata( 'post', self::$post_id );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( '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', 'flight_number', array() );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_number' );
+		$meta = get_registered_metadata( 'post', self::$post_id, 'flight_number' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( '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', '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' );
+		$meta = get_registered_metadata( 'post', self::$post_id, 'flight_number' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( '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', 'flight_number', array() );
 		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
 
-		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_pilot' );
+		$meta = get_registered_metadata( 'post', self::$post_id, 'flight_pilot' );
 
-		unregister_meta_key( 'post', 'post', 'flight_number' );
+		unregister_meta_key( 'post', 'flight_number' );
 
-		$this->assertWPError( $meta );
+		$this->assertFalse( $meta );
 	}
 
-	public function test_get_registered_metadata_invalid_object_type() {
-		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
-		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+	public function test_get_registered_metadata_invalid_object_type_returns_empty_array() {
+		$meta = get_registered_metadata( 'invalid-type', self::$post_id );
 
-		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
-
-		unregister_meta_key( 'post', 'post', 'flight_number' );
-
-		$this->assertWPError( $meta );
+		$this->assertEmpty( $meta );
 	}
-
-	public function test_get_registered_metadata_invalid() {
-		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
-
-		$this->assertWPError( $meta );
-	}
 }