Creating new accounts with Unicode characters in the wiki username succeeds on Wikitech and leaves the user logged in and able to edit. If the user ever becomes logged out (because of session expiration or deliberate logout), they will be unable to log back in.
Verified by creating and using [[https://wikitech.wikimedia.org/wiki/User:T%C3%A9st_%C5%91f_Un%C3%ADc%C3%B8d%C3%AB_%C4%AFn_Us%C4%93rn%C7%8Em%C4%97|User:Tést őf Unícødë įn Usērnǎmė]]. Account creation worked as expected. Subsequent login attempts fail.
Partial workaround: password reset will let you log into Wikitech (since it completely circumvents LDAP authentication). Does not help with Horizon.
When I try to log into Wikitech as `Gergő Tisza`, I get `Incorrect username or password entered.` (it doesn't even ask for the 2FA token, so it's not an issue with that). On Horizon it's `An error occurred authenticating. Please try again later.` On other LDAP-login-based services like Logstash the same password works, though. I haven't changed the password lately; I don't think somebody else changed it either, since that would log me out of Wikitech, which did not happen.