I can see why you would think that, because you are probably picturing the people who would use this feature as entitled white ladies who put too much faith in Gwyneth Paltrow's take on medicine. In truth, they are people with conditions like MCAS who need to travel and are grateful for any accomodations that aren't a total assault on their bodies. Because of that, as long as the host makes the accomodations the say they will (e.g., no scented products and cleaning with safer products) they can expect very positive reviews.
For example, the only fragrance free Airbnb I know of is near Sedona, is always booked, and has great ratings.
In my own experience, I have managed to find a number of hosts who were willing to work with my medical issues, and we have always been on good terms because the accommodations I asked for were relatively easy (see above) and even though I wasn't perfectly comfortable, I felt much better then I would have in an ordinary hotel or Airbnb. But a fragrance free filter would have saved me SO MUCH work contacting hosts and explaining my medical needs.
The only time I have ever heard of a person leaving a bad review was when a woman was repeatedly assured by a host that the room used unscented laundry detergent and no scented air fresheners, which turned out to be completely untrue.
As for the other points like the healthy bed (i.e. A bed that doesn't off gas the VOCs that trigger our symptoms), those are things that would make a space even better for many, but I don't think anyone would expect those things just because a place is listed as fragrance free.
I think you are also underestimating the prevalence of these conditions because people with them usually don't mention them. People who report any degree of problems with fragrances are about 20% of the population (mostly headaches and such) and severe sensitivities are about 2%. That means a number of hosts may already be practicing fragrance free cleaning of their properties, but are unable to communicate it with the large numbers of clients who would love to rent such a space. At one rental I checked out in 2021, I spoke to a cleaning lady and asked if she could use mostly vinegar, hydrogen peroxide, and baking soda to clean. She replied that those were all she used because her son was "sensitive to chemicals."
