Bumping old threads is not, in itself, against the rules. We don't have hard-and-fast rules about it. But it can be annoying if done for no very good reason. In particular, it's odd to jump into a years-old conversation as if it's ongoing.
The following is not exhaustive.
Good reasons for bumping an old thread:
It's about some project you were working on, and now you have an update for it.
It's about some more general topic which is back in the news now, or a story which has been updated.
There's some kind of deadly misinformation in the thread which you really need to correct.
There's an important link, perhaps to Civ-related downloadables, which is broken.
Bad reasons for bumping a thread:
You happen to have come across a post saying something you disagree with, and you want to post a rebuttal.
You happen to have come across something you like, and you want to post "Great unit!" or similar.
The following is not exhaustive.
Good reasons for bumping an old thread:
It's about some project you were working on, and now you have an update for it.
It's about some more general topic which is back in the news now, or a story which has been updated.
There's some kind of deadly misinformation in the thread which you really need to correct.
There's an important link, perhaps to Civ-related downloadables, which is broken.
Bad reasons for bumping a thread:
You happen to have come across a post saying something you disagree with, and you want to post a rebuttal.
You happen to have come across something you like, and you want to post "Great unit!" or similar.