/**
 * Callback function for the 'webmention_avatar_directory' filter.
 *
 * @param mixed $upload_dir 
 * @param string $url 
 * @return mixed The filtered value.
 */
function my_webmention_avatar_directory_callback( $upload_dir, string $url ) {
    // Your code here.
    return $upload_dir;
}
add_filter( 'webmention_avatar_directory', 'my_webmention_avatar_directory_callback', 10, 2 );

apply_filters( 'webmention_avatar_directory', $upload_dir, $url )