Gravatar support is included by default. Their default icons are similar to Identicons.
You could also integrate Robohash by using an advanced theme and overriding the avatar() function. Get the username and output an image with that, eg https://robohash.org/Scott.png
Probably better to use a hash of the name just to be safe. Something along the lines of
public function avatar($item, $class, $prefix=null)
$hash = sha1($item['who']['data']);
$item['avatar'] = '<img src="https://robohash.org/'.$hash.'.png">';
parent::avatar($item, $class, $prefix);