At least this wasn't just some "OMFG NERF IT!" post. It still won't work though. Pantheon will ALWAYS fall off later with his kit... by depriving him of the one thing he does well you will leave him with nothing. Pantheon is currently strong for a LOT of reasons and you can't just slap a simple fix on him to make him balanced.
The game has been far more snowbally for a while now, and matches tend to end much, much earlier these days. This was the first thing to give Pantheon a push into a good winrate. He is an early game champ, after all.
After the grossness of the tank meta left, top lane became a lot more.... squishy. This puts Panth against a lot of people he is very strong against because he is a top-tier 1v1 dueler and early game monster.
Lethality was a double-boon for Panth. Now that it's buffed back into OP status it is helping him twice over. Not only are lethality items REALLY good on him, but everyone is spamming squishy AD champs and building lethality on them. Meaning his lane opponents are usually fellow AD champs with no early defensive items. Is it any wonder that he's stomping them?
Pantheon does exactly ONE thing in this game. Win lane 1v1. He doesn't snowball off it really... he falls off pretty hard. He just dominates early lane. By crippling his damage you would leave him utterly useless for the entire game, because barring a rework his kit just isn't worth anything much outside of those early 1v1's. What's more, you'd be nerfing him because of the items and the meta making him strong. When those things inevitably change he will be even MORE garbage if he's been nerfed previously.
I understand he's a bit oppressive now, but we need MORE champs like Pantheon, not LESS. He's a specialist with a very defined set of strengths and weaknesses. Now every freakin' champ needs to be some high skillcap nonsense full of dashes, % damage and complicated, overloaded kits. Are you against Pantheon? Ok cool, life will suck for you for a while. Play safe and outscale the mofo. Did you pick a squishy lethality abuser and he counter-picked you? Well, that's kinda your own fault then, eh? If you're gonna go something like that top and you don't ban him you're asking for it. There are five bans now.... if you can't handle laning against someone but you REFUSE to pick champs that can handle them better AND you don't ban them... I'd say your death is your own fault. Like some 0/4 Riven who KEEPS trying to all-in the fed guy that's beaten her 1v1 repeatedly... you aren't correcting your mistakes or altering your strategies at all... of COURSE you're going to keep failing.