Why should they be allowed?

Because we live in Unicode era and there are languages which use not just ASCII letters? Чесно!

As a usecase or whatever you call it:
On Wikimedia Phabricator installation users who have their onwiki username in non-ASCII scripts have to use a different username on Phabricator and it is partually more difficult for them, partually more difficult for others (when you want to subsribe a certain person in this case you've got to know what username they have on Phab instead of using their normal one)

Is there a downstream task for this request?

I am not aware. Shall I create one?

T4267 is probably the upstream fix for the core issue at hand.

Aaannd I think https://phabricator.wikimedia.org/T877 for the downstream. Would that resolve your problem?

Hmm, well if those T4267's alias accounts would support non-ASCII then yes for this case. But I am unable to see why not to complete this one :)

But I am unable to see why not to complete this one :)

See Contributing Feature Requests.

We've already declined support for this. See T6003 for discussion.

I couldn't find that task, actually. I searched for 'username' and closed, and I just double checked again and it still didn't appear?

"username" doesn't match "usernames" until T6740.

It's the first hit for me if you search for "usernames".

ah I guess it matched on 'username' elsewhere, it gives the impression that is matched on 'usernames' though since it highlighted it as usernames

Yeah -- the highlighting code is more flexible about what it matches than the actual search code is, which makes T6740 worse / more confusing.