@OldandConfused I don't see a large problem here: you can delete your toots, you can change your credentials and then you are as good as gone.
Deleting a large number of toots should be possible via the API (didn't try deleting from there yet) but actually, if you posted here, it's public. So think before tooting wouldn't be badadvice, even if you could delete an account.
@cynix My problem is this should be basic functionality, not my posted toots. I prefer when I choose to leave a service my account dies. I don't think this is a particularly controversial stance really.