Excluding existing archiving sites, do you have an expectation or think that data stored on lemmy should be permanent? E.g last 20+ years. Would it matter if old inactive content was culled? If you had the ability to export data if a sever was due to go down, what would you actually do with it.
The best way to get both data permanence and right to be forgotten (and also protection from censorship), is data sovereignty.
The master copy of each user’s posts should originate at the client.