What about migrating historic data from Reddit ? Is that possible?

From a technical standpoint: Archive.org is archiving Reddit data. So someone just needs to write an importer for that data. I would have suggested reading from Reddit's APIs directly but they are shutting those down.

From a legal standpoint: I am unsure.

Do they archive nsfw too?

Edit: I checked, and Archive.org seems to be snapshotting once per quarter or less. Not a realistic strategy for any sub with even modest traffic.

Or legal?

I'm certain you got downvoted over the word 'serf'. I upvoted because my reaction to that framing is more positive, but it's not a framing I would use, myself, because I would expect to get some flak for it.