If the reason for deleting your account is something related to your privacy then send a message to @Theymos, and if your reason is sufficient, your account will be deleted (all posts will be deleted and the topics you created will turn anonymous)
Forum moderators, admins can nuke accounts of spammers, scammers. That is a little bit similar to delete an account but at least user account still exists.
Admins can change an original username to a random username. I saw some cases like this with privacy issues like you mentioned.
Changing an account to Anonymous is very long in the past and I did not see any case recent years at least since 2017.
As you can see here, I only change the username to "Anonymous" on request.
Usernames list from Loyce.club with
does not exist for deleted accounts.