This just makes no sense. You can't paint a message on a billboard and then get angry when cars driving by can read it. If a group of people wants to post messages secretly amongst themselves, the Fediverse is the wrong place to do it. Use Telegram or a private Discord server or something, but even then screenshot leaks are bound to happen eventually.
If you are truly concerned about the safety of "vulnerable folks," you should be all for educating them on the true workings of the internet so that they can avoid publishing things they don't want the world to see. You are saying it's fine for them to use a system whose stated purpose is blasting messaged all across the internet to whoever wants to see them and then expecting all the recipients to know about and follow the unwritten rules they are somehow bound to when handling that message. That is not a realistic expectation.