Auto-deleting posts has the problem of destroying any future benefit. In my opinion, the greatest benefit of Reddit is the ability for the public to find answers to niche questions but sharing discussions. Every single person with a problem for looking for an opinion, doesn’t have to find relevant people to ask anew for an answer.
Again, if someone wants to have a private discussion that people can’t just look up, I question why they would Lemmy at all. Something like Matrix or Signal is far more suited to that goal.