The mentioned user (as you’d expect) gets a notification anyway so they’d be able to see if they’ve been mentioned. I’ve never missed a mention notification.
Thank you, @ondrej! It all sounds reasonable, but I just had a situation the other day when a user didn’t realize they were mentioned as their username was “user99999”
Probably it’s quite a rare case, but still, it would be nice to have one’s username stand out in a post.