It would be a shame to lose the wealth of knowledge with easy-ish search that subreddits like datahoarder provide if the subreddit is taken down or stays locked forever. Sure it is currently accessible, but will it stay that way?
I know it is being archived, but the accessibility part is the problem.
I think it would be very hard to argue in court that someone’s ideas and thoughts that they made belong to reddit just because they posted them there. That is also why you can request reddit delete all your data and they must comply.
As for the legality of taking those comments and posts. I don’t know for certain. The internet archive already does though. If anything, they would have to remove any content that a person wants removed that they made. Like a DCMA request.
Like with most things on the internet, if it is illegal and nobody is enforcing it, it might as well be legal.