I run a private Mastodon Instance. The server gives me the option to automatically delete the media-cache after a given number of days (I have it set to 180 days). If the data is needed again, it will be pulled from the original instance. Again. If still available.
An additional setting is the deletion of the federated content at all, which is saved on my server. This probably cannot be reverted, as the server doesn’t know anymore what to pull from what server…
I have this setting at 365 days.
Additionally I can set my personal account to auto delete my own posts after a given time. With being able to set some excludes (favorited, bookmarked, posts which got a minimum of likes, …). But this is not a server setting, it’s an account setting.
Social media does not need to be saved forever. :)
And this stuff does need to be saved forever. The biggest advantage of reddit was that you could Google anything and find a detailed reddit post about your issue/topic from 3 years ago.
I always called reddit anti-social media since it is anonymous by default, and few people choose to fully de-anonymize themselves. That's one of my favorite aspects about this style of communication - while obviously some people still managed to get their ego involved, and that can happen even in brief throw-away interactions like 4chan, it's not nearly as bad as facebook or twitter where online behavior and interactions are tied to 'real world' identities.
Basically, data storage is cheap. Currently it can cost as low as $0.01/day for other instances to store the content that is being sent by the largest communities. Otherwise instances can clear out old data and let the larger instances be the archive hosts.
I could imagine a scenario where older posts expire from local instance caches (except for possibly a headline?) and if it comes up in search it’s pulled from the original instance again. I could even envision a scenario where pictures, video, and even text posts expire after a certain amount of time from their original instance (or after a certain amount of time with zero interactions). I could also see mods being able to flag content as important, or maybe after a large number of votes it automatically becomes important, and prevents removal from the instance.
All of this would be configurable by the instance owners of course.