I don't think Mods appreciate reporting long-forgotten posts:
Depends how far back you decide to go, if you see a shitpost on the same page where a discussion is still being held (e.g. not a very active topic, or one revived recently), I don't see any wrong in reporting an older post, as other users visiting or returning to that topic may otherwise waste their time having to skim through the shitpost(s).
Strangely, these posts have been marked as bad. Other high-spam users have also been cleared for similar posts.
These posts are shit, but they're borderline cases, so you might not necessarily see them removed.