There is no way to guarantee anything has been deleted. There is only, "I can't see it anymore, so I hope that means they aren't keeping a copy somewhere." Nostr is no different, it's just more honest about it. You can "request deletion," but that's as good as it gets. Even so-called ephemeral events are only ephemeral if the relay respects the expiration and deletes the note. Owncast users are fooling themselves if they think they have any guarantees that their chat messages are deleted by Owncast servers any more than by Nostr relays. And if you could so easily build a brige that pulled in Owncast chat messages, anyone else could do so in order to keep a record of the chat messages too.