He's been sitting at a 54-55% winrate now for the last several patches. This isn't an outlier, it's not just his "mains." He's sitting at 7% pickrate, which is moderately popular especially in top lane. He also doesn't really "fall off" until 40+ minutes.
Pantheon as a lane dominator used to have a very clear avenue to be defeated: outsustain his mana pool. Between corrupting potion, meditation mastery and thunderlords, Pantheon wins almost every top lane match-up with his mana costs not really being a problem anymore. Yes, there are some champions who beat him but that doesn't justify his ridiculous winrate and how effortless it is to stomp lane AND ROAM with a semi-global ultimate. This champion has needed to be reworked for a long time, his design is ancient and a product of a very different game where matches were won and lost based on counterpicks. League has obviously gotten away from that. There should be more to lane than eating autistic spear after autistic spear but that's literally all Pantheon does.
As for nerfs, I would honestly suggest increasing the cooldown on his Q a bit and perhaps reducing execute crit passive damage on his E. I don't mind the idea of Pantheon being a lane bully but not to the extent that he is and not with the very little effort and interaction required to go about it. If he wants to bully, he needs to actually commit more than just spears. He needs to risk his tail and actually make use of his passive rather than the lane just be the following series of events:
have him passive block
haha you're dead
And yes, I did just play against pantheon and yes I will admit a clear and obvious bias. I did not lose but I fucking hate this champion and he is one of the worst designed champions in the game. Laning against him is miserable, I don't care if he "falls off." Riot has gone away from champions who dominate lane like that for a reason, because people hate playing against that shit. It will be inevitable that somebody will throw out "haha just play champion x and make him cry." Okay and what about the thousands of other times he's used to counterpick you and 90% of the top lane roster? Furthermore, his statistics are simply too high right now. He needs nerfs plain and simple.