I recently discovered to my surprise that Co-Star allows unicode in their @usernames and this is the first network I have seen that has this support. I have been complaining about the fediverse lack of unicode support in usernames for a long time now and its interesting to see centralized projects get to this milestone first. Though its probably much easier to get this to work right in a centralized environment.
an example to show it in action, notice the á near the end of the username