Is it ever okay to post on an old topic?

Is it ever okay to post on an old topic? Like if your post adds to the discussion, or something like that?

Depends, it has to be helpful and related to the topic

(a lot of them are offtopic though)

Probably not in HwS, or BaG. I think it's allowed in the Snap Suggestions category though.

I'm saying this because I would assume that some rules about forum ettique come over from Scratch, because both forums have general similarities in what you should/shouldn't do

It's not a question of how old the thread is. It's a matter of not saying things that have already been said. So read the entire thread before you post to it. And if it has one of those solution checkboxes in its title, be extra sure you have something to add before posting. That should be the case regardless of the age of the thread.