Steve Rogers was disabled before the serum, but I think there’s a fair argument to be made that he was still disabled after the serum, too.
Like sure, he’s not disabled in the sense that he’s able bodied and average height and has working lungs and no chronic illnesses, but he’s disabled in the sense that the world is not built to accommodate super soldiers. He needs extra rations. He can’t take normal pain medication. Gym equipment and daily appliances aren’t built to accommodate his super strength. And what about when it comes to exerting non-lethal force? Can he still do delicate fine motor tasks? Stretching a little further into headcanon, one could even argue that Steve’s eidetic memory and super senses are also potentially disabling. Sure, they’re useful for battles, but I bet it’s overwhelming, too, to be taking in so much information all the time. At the very least, his experience as an “enhanced” person is an interesting allegory.