If you say you have an underlying health issue that prevents you from wearing a mask, they can't keep you out or ask what that health issue is.
If you want to, but just stating a health condition is enough. Just keep the "I'm not going to wear a mask" part to yourself at that time.
They will, but unless they are your doctor or lawyer, they can't ask what your health condition is. My health condition is masks make me sick to my stomach because it's all a big lie. But that's between my doctor and me…………if I had a doctor.