// Get posts with no thumbnails
//add_action('wp_head', 'sz_posts_without_thumbs');
function sz_posts_without_thumbs() {
      $args = array(
        'post_type'      => 'post',
        'post_status'    => 'publish',
        'posts_per_page' => -1, 
        'fields'         => 'ids',
        'no_found_rows'  => true,
        'ignore_sticky_posts' => true,
        'meta_query'     => array(
            array(
                'key'     => '_thumbnail_id',
                'compare' => 'NOT EXISTS',
            ),
        ),
    );

  
    $news_query = new WP_Query($args);
    $news_post_ids = $news_query->posts;
    echo "<!-- SZCHECK Posts without thumbnails URLs: -->\n";
    if (!empty($news_post_ids)) {
       
        foreach ($news_post_ids as $post_id) {
            $post_url = get_permalink($post_id);
            echo "<a href=\"$post_url\" target='_blank'>$post_url</a><br>"; 
        }
       
    }
}