As far as I'm aware all posts of any permabanned users are deleted these days. Haven't really kept on eye on who's getting banned much, but it seems that way from the little do know at least.
Although even if it were just spam bots/alts, my 2nd point still stands. VGC isn't good at dealing with deleted posts. It messes up threads, unread messages and probabyl some other stuff. Now when that was an issue (in a only a couple of places I might add) because ioi was cheating to get a milestone post that was a different matter. For starters that was ioi, but also it was kind of a funny reason and a one off. It was contained to one (or a select few?) thread and that's it. Seems much more widespread now and without the funny back story.
We don't delete posts outside of spam accounts.
Unless the user requested it, which has only happened once to my recollection in the past almost 2 years, and that was Nightly Poe.
And in that case an Admin takes care of it.