You mean modern Blizzard does that. Oldschool Battle.net had a 16 alphanumeric characters with underscore allowed and that was that. At least for Warcraft 3 it

You mean modern Blizzard does that. Oldschool Battle.net had a 16 alphanumeric characters with underscore allowed and that was that. At least for Warcraft 3 it's been the case since 2002 for a very long time (until quite recently last year they allowed fancy symbols in usernames). You also had to login at least once every 3 months or Blizzard purged your account.