Ticket #23983: 23983.2.diff

src/wp-includes/post-thumbnail-template.php

@@ -57 +57 @@
                 return false;
         }
 
-        return (int) get_post_meta( $post->ID, '_thumbnail_id', true );
+        $thumbnail_id = (int) get_post_meta( $post->ID, '_thumbnail_id', true );
+
+        /**
+         * Filters post thumbnail ID.
+         *
+         * @since 5.9.0
+         *
+         * @param int|false        $thumbnail_id Post thumbnail ID or false if the post does not exist.
+         * @param int|WP_Post|null $post         Post ID or WP_Post object. Default is global `$post`.
+         */
+        return (int) apply_filters( 'post_thumbnail_id', $thumbnail_id, $post );
 }
 
 /**