It seems that what you would like is something like 4chan, where the post will get deleted if it’s not popular. But even that, there is no way to prevent data harvesting. If it’s public, then it is public. There is nothing you can do about it. Encryption wouldn’t solve anything either because you want this data to be read by everyone so you cannot really encrypt it.

The fediverse is kind of the same as a public room where anyone can come in and just listen, take note, see who is talking and respond in the same way.

This is the point of social media. If you don’t want to participate in it because of privacy, then don’t and just lurk (or listen) like most people do.

By definition, if it’s on the internet, it’s pretty much there forever. People need to be careful on what they share on the public space, in the same way you would when talking to a big crowd. You are not talking with your friends here, you are talking to the world. If you are any privacy, you just cannot have it here. That’s impossible.