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.
Even if it didn't, that would be trivial for anyone to do with the API. If you're saying things you don't want people to know you said, don't use your name. Posting public, discoverable content is the entire point of Lemmy. Hiding what you're doing wouldn't solve the problem.
I think Musk's individual recklessness ruined Twitter a lot more than any corporatism.