I got in on reddit super early. Every few weeks I get a message from someone referencing a comment a decade old, now either featured in some youtube video or coming up in their google search. It's unnerving shit and I check my politics there a lot more than I do here. Just as a matter of user safety it's good that our posts disappear after a month or whatever. Maybe it's eventually a few months or a year, but if we ever return to the normal model of forums preserving everything forever it's a big infosec risk.
Tagging posts for preservation is a good idea. I intentionally want my dog's subreddit to last and don't care if my modernism posts get archived.
I have no idea about programming but perhaps a week before the wipe we could have an announcement post for folks to nominate threads to be preserved from that round of deletion. No idea how much work that is to implement though.