I suspect the percentage of people able and needing to claim in this country is rather higher than the percentage of people not wearing masks. People are averse to confrontation and, having been relentlessly told that masks help are afraid of infecting others, so they are just not going on public transport or in shops when they can avoid it. Certainly I'm in that position.Trying desperately not to take sides in this debate, but from an observational perspective it’s interesting to see the differences in attitudes (both official and public) in various countries. We are fairly relaxed, with no need to ‘prove’ anything. But Germany, for example, has no similar list of exemptions. In the US, transport providers got fed up with the government not formalising any rules, so made their own. All major airlines there enforce a strict ‘mask over mouth and nose’ policy, and don’t accept anyone for travel who refuses (for any reason, whether we would allow an exemption or not). The only exception is Delta, who allow maskless travel but only following a video consultation with their company health department, who will assess the claimed exemption. And in neither of these countries is there a huge level of non-adherence, at least in a transport setting. It seems allowing exemptions increases the incidence of people claiming them.
Obviously if they started mandating masks in even more inane places like outdoors then "compliance" would probably drop, because these people have not actually stopped existing for the convenience of legislators. Once people are not wearing a mandatory mask outdoors they would also likely stop avoiding places where they were supposed to wear them indoors, so the effect could even be outright counterproductive.
As to the situation in other countries, people may be staying home even more. I'd say that from what I've heard in most "no exceptions" countries, compliance is actually lower than here, and enforcement is even patchier. In particular there is a lot of tokenistic wearing for a few seconds, or not over the mouth going on in some countries.
As regards air travel, well over half the population of most countries don't get on a plane in any given year anyway, and this year has hardly been a good year for going abroad, so there's really been no need. If you have an exemption then far better to get a train and claim it.