They are certainly allowed, but they risk not being taken seriously if they flip-flop a bit too often
Sounds like you're not on the winning side of an argument.
As to me, your proposal to introduce the "banned" rank is simply incomparable
In respect to the amount of devastation and damage that it would have caused and made if it had been accepted.
Damage to account farmers, signature spammers and the people who cry when their post count goes down?

As I told, I don't see any reason in deleting old threads apart from a rogue moderator starting a vendetta against a certain user.
Number of times that this has happened: 0. The value of solving issues which do not exist is, exactly 0.
Ouch. My post count tanked by ~1000 due to this?
Enjoy the pain as they would say. It's not a big deal, right?
It is irrelevant for me, but I wonder whether I am the person who lost the most amount (absolute wise, not percentage wise).