This is proper etiquette unless they blatantly and obviously look like they need a seat in which case you are simply expediting an almost certain necessity.

I hate being asked when I’m tired and barely verbal due to pain, but a non-answer, shaking my head, simple “no” or sorry should get the point across that I need the seat.

What’s not ok is questioning someone’s no for “not looking like they need it” when they are specifically in the disabled zone. I’ve had other disabled people harass me terribly. It’s sick and I would never do the same. I consider those people scum. I once had an elderly dude passive aggressively talk shit about me because I wouldn’t move for his wife who needed a seat (didn’t seem concerned for himself). He was saying that young people have no respect for elders. They found a seat close to me because someone else moved to open up two side by side seats, in which I heard the wife suggest (accurately) that I probably needed the seat and how awful it is to be young and disabled. Bless that empathetic woman. It seemed to dawn on him and I could tell he felt bad cause he calmed right down. My cane was tucked against the seat and not obviously visible until I got off.