Polearms are probably my favorite weapon, but I see a lot of people saying that it's damage is irrelevant later on. I was wondering if that was still the case considering the posts are all about 1-2 years old.

Yes. IMO, it's also one of the easiest weapons for early to mid game because it has easily craftable magatama that are useful even at level 115 if you haven't made Thunder Breakers yet. It's true that its raw damage output is often subpar compared to other weapons but it's still viable even late-game.