I have also thought this is a good idea. I think that the ActivityPub standard should have a required field that lists a copyright license. Then a copyleft style copyright should be created that allows storing and indexing for distribution via open-source standards, and disallows using for AI training and data scraping. If every single post has a copyleft license then it would be risky for bigtech to repurpose it because if a whistleblower called them out that could be a huge class action suit.

A good question is if a single post can be copyrighted. I think it could. Perhaps you would consider each post like a collaborative work of art. People keep adding to it, and at the end of the day the whole chain could function as a “work”. Especially since there is a lot of useful value and knowledge in some post threads.