add_filter('rank_math/frontend/robots', function($robots){

  if ( ! is_singular('photo_sets') ) {
    return $robots;
  }

  $post_id = get_queried_object_id();
  if ( ! $post_id ) {
    return $robots;
  }

  $indexable = get_post_meta($post_id, 'photo_set_indexable', true);

  if ( $indexable === '0' ) {
    return 'noindex,follow';
  }

  return $robots;

}, 20);