Redirect /%category%/%postname%/ to /%postname%/

// REDIRECT /%category%/%postname%/ to /%postname%/ (for old posts links in social, after change permalinks)
add_filter( '404_template', 'custom_redirect_to_category' );

function custom_redirect_to_category($template) {

    if ( ! is_404() ){
        return $template;
    }

    global $wp_rewrite;
    global $wp_query;

    if ( '/%category%/%postname%/' !== $wp_rewrite->permalink_structure ){
        return $template;
    }

    if ( ! $post = get_page_by_path( $wp_query->query['category_name'], OBJECT, 'post' ) ){
        return $template;
    }

    $permalink = get_permalink( $post->ID );
    wp_redirect( $permalink, 301 );
    exit;
}