getimagesize() will trigger a PHP warning if it is unable to access an avatar. This might happen if the image can't be reached, i.e. by a 404:
[phpBB Debug] PHP Warning: in file [ROOT]/includes/avatar/driver/remote.php on line 96: getimagesize(https://www.phpbb.com/avatar/55502f40dc8b7c769880b10874abc9d0.jpg): failed to open stream: HTTP request failed! HTTP/1.1 404 Not Found

In order to prevent this, the @ should be added back.