From what I can tell, nothing in what @fiatjaf suggested can in any way be characterized as a guarantee of deletion. Only a best effort to ensure deletion and a "trust me, this is what we're running on our relay, here's the open source code that shows how it works allowing me to 'safely claim that messages are not persisted,' but there is no way for me to prove that neither I or anyone else is 'malociously persisting them.'" Indeed he also acknowledges that the same lack of guarantee would also be an issue with messages residing on OwnCast servers. Owncast has passed UAT for them only because they never bothered to ask whether Owncast can provide guarantee that their chats are deleted, and Owncast never bothered to be proactively up-front that it's impossible to provide any such guarantee.