I've asked about that before - but I think I've found one reason why I might report old posts, especially if:
The warning above is very clear that moderators do not moderate old posts for any reason.
No, that's not exactly what it is written. If there is a
very good reason to report an old message (malware, doxing, etc...), report it and a mod will handle it (normally).
Spam posting in old threads that are several years old that may have been buried deep and doesn't seem to bother forum and readers anymore. So that might be the reason why mods don't moderate old posts.