@@ -629,6 +645,7 @@ function _list_meta_row( $entry, &$count ) {
  * Prints the form in the Custom Fields meta box.
  *
  * @since 1.2.0
+ * @since x.x.x The default list of meta keys used to populate the dropdown was removed.
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
@@ -651,24 +668,7 @@ function meta_form( $post = null ) {
 	 */
 	$keys = apply_filters( 'postmeta_form_keys', null, $post );
 
-	if ( null === $keys ) {
-		/**
-		 * Filters the number of custom fields to retrieve for the drop-down
-		 * in the Custom Fields meta box.
-		 *
-		 * @since 2.1.0
-		 *
-		 * @param int $limit Number of custom fields to retrieve. Default 30.
-		 */
-		$limit = apply_filters( 'postmeta_form_limit', 30 );
-		$sql   = "SELECT DISTINCT meta_key
-			FROM $wpdb->postmeta
-			WHERE meta_key NOT BETWEEN '_' AND '_z'
-			HAVING meta_key NOT LIKE %s
-			ORDER BY meta_key
-			LIMIT %d";
-		$keys  = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%', $limit ) );
-	}
+	assertNotEmpty($keys)