To add on to Pipsqueak’s comment, there is an official FAQ, and it does mention “necroposting” or “reviving dead topics”:
Do not necropost topics that are concluded. If you want to continue the discussion or have additional input on an old topic, that’s ok!
(emphasis mine)
So, pretty much what Pip said: It’s a bit of a judgment call, but I’d say that if the old topic is still relevant, and you have something productive to add (not just “I agree with this” or “This is silly” or “When is <insert character here> coming?”), then there’s nothing wrong with replying to a long-dormant topic.
For example, if you find a topic from three years ago with people making suggestions to improve Heist, and you have a new suggestion, then that would be fine, as Heist hasn’t been touched during that time so all the earlier suggestions are still relevant. If, however, you want to talk about war matchmaking and MMR, it wouldn’t be good to resurrect a topic from last year because the MMR and matchmaking systems have been changed since then.