One defense against it might just be to actively crawl Facebook and externally record the contents of posts as soon as they're posted. Then you have a record of everything that got deleted.
I don't know how you scale that up to make it easy for everyone to find "disappeared" content on any platform. Maybe some kind of peer-to-peer system where everyone's browser cache basically acts as a searchable archive, with a browser plugin that inserts a button into web pages to show disappeared content.
(It's also worth noting that probably a lot of content that was removed by moderators was removed for a legitimate reason. So, ideally you'd have some sort of crowd moderation to get rid of the stuff that really is spam or hate speech or whatever.)
I don't know how you scale that up to make it easy for everyone to find "disappeared" content on any platform. Maybe some kind of peer-to-peer system where everyone's browser cache basically acts as a searchable archive, with a browser plugin that inserts a button into web pages to show disappeared content.
(It's also worth noting that probably a lot of content that was removed by moderators was removed for a legitimate reason. So, ideally you'd have some sort of crowd moderation to get rid of the stuff that really is spam or hate speech or whatever.)