If all of a user’s posts are deleted keeping their username when quoted in other posts wouldn’t necessarily make a forum any less difficult to read, would just mean new readers wouldn’t be able to tell who was being referenced other than random number set to anonymized user account.
This does seem like quoted username should be deleted as part of account deletion, maybe the default could just change that to “deleted user” or something unless account is anonymized to keep user posts? That would be more difficult to understand what people are talking about if all deleted account usernames are just changed to “deleted” instead of a number that is the same for all their posts / quoted username.