Commenting Question

So I was just wondering about people’s opinion on commenting on older fics, or older chapters in a story. I’m not certain on if that’s okay or not. On one hand you can search on sites like AO3 and Fanfiction.net based on the number of comments, so like Kudos it could promote the work. On the other hand sites like Spacebattles specifically have rules that state you can’t comment on older fics. So presumably enough people found it annoying. I don’t want to bug authors over something they’re not really interested in from four years ago. Finally if I’m joining an on going fic with a lot of chapters I might have a thought or reaction in an earlier chapter but is it okay to leave it there, or should I just ignore it since the fics moved on. Odds are it wouldn’t make sense to add this thought from chapter three in chapter forty three.

So anyways, what’s the protocol?