You need to stop banging on about this. No-one wants to wear something that says LOOK AT ME, I'VE GOT AN EMBARRASSING MEDICAL PROBLEM!
As there are many bases for exemption that aren't embarrassing (asthma, for example, if a mask affects your breathing adversely), then such a thing would not have that effect at all.
12% of the population have some form of asthma. That's a lot of embarrassed people!
I'd be inclined to respond with "mind your own business". That's neither a yes or a no.
That would trigger me walking away then mentioning to shop staff as I paid for my goods, as it tends to be a response that comes from people who are deliberately being awkward (a bit like the classic "no comment" in a police interview). Of course it's then up to the shop staff what they do or don't do.
I think it's *extremely* disrespectful to go probing into what may well be others' medical business.
The question isn't why they are exempt, just if they are exempt. Indeed it wouldn't even be framed as a question, more a trigger said as a parting shot while walking away - someone who has a guilty conscience would attempt to escalate that, and that's when I would be "triggered" to report the matter to staff. Someone who was actually exempt would mostly just mumble and continue their shopping.
It's like the airport luggage "did you pack your bag yourself" question. Fairly pointless in many ways, but the nominal basis of it is that if you genuinely did you'll just say yes, confidently and clearly, but if you didn't you'll mumble and they will know there is a need to probe further.