Usually those artist did get some recognition during their life, but never got into the main stream. That changed due to the main stream changing and the people who did like the art showing it again. That is actually rather easy to do with something like the Fediverse. It just requires a download option. Especially when everybody is aware, that the content will be deleted, that would be a decent option.

Also a lot of content on social media in general is very short term. Stuff like politcal discussions are fairly useless after a few months in most cases. So that can be deleted without much care and again, if somebody wants to preserve it, they easily can just download it.