WP_AJAX-ClearOrphanMetaAction.php

<?php

Class ClearOrphanMetaAction extends WP_AJAX{

    protected $action = 'clear-orphan-meta-action';

    protected function run(){

        global $wpdb;
        $wpdb->get_results("
            DELETE meta
            FROM {$wpdb->postmeta} meta
            LEFT JOIN {$wpdb->posts} posts ON posts.ID = meta.post_id
            WHERE posts.ID IS NULL
        ");
        
    }
}
ClearOrphanMetaAction::listen();