With regard to recent cases of deleting hundreds of posts from some users (justified, but still unnecessary), and banning users for something they did 4-5 years ago, I would suggest that posts should be reported to moderators max two years from time they are posted. I think it would be quite fair solution for everyone, if user is not punished in two years after posting something, it should remain in the past.
Some will certainly disagree with this proposal, but I think we need to focus more on the present or the recent past.
How would you really police this effectively though? Do you think it should be made a forum rule or just an unwritten one? Should we prohibit old posts from even being allowed to be reported in the first place by removing the report button for posts that are x amount of time old? Then what happens when there is something that should be removed like doxxing or whatnot? I agree that half of the time very old posts aren't important (especially when people are seemingly going out of their way to find minor infractions) but the only way I can think of is to make it known that it's unnecessary to report certain things, especially if they're historic cases (and for minor violations). Probably wont stop people from reporting them though. Probably bigger things to worry about as well really.