I don't know since when forum did not allow account delete, but I guess when it became bigger enough to let admins think that account deletes might result in complicated situations related to those accounts. Let imagine someone create an account to manage a project, and create OP; then months later delete that account and there is no proof left if forum does not have regularly archival procedure.
One more thing to consider, by the way, is threads can not be deleted.
I don't know that at beginning threads allowed to delete or not, or that restriction was implemented nearly same day or same period in which account delete disabled.
There was an unofficial account delete service, but suspended years ago - since May 2015:
[SAD] I delete your accountAccounts do not actually get deleted from the database, admins could restore an account.
For now, people only can ask for username change, but it is not for all, and only very special cases (VIP, donators, or very very special case of normal user) will get acceptance and support from admins to do username change.
Done. Now only moderators, Donators, and VIP members can change their names automatically. Other members should contact me. I'll probably ignore your request if you have less than 50 posts (unless your posts are very good).
Just a few months ago, there was a user - at Senior Member rank, that was reactively after years, and asked for username change, because he did not have attention to keep using that account, and did not want his personal name abused (that is what I remember). Then, admin or global moderator gave him a hand, and changed username after few hours (from his personal name to ABCDEXYZ, something like that)