I can imagine needing a medical certificate stating that I had an allergy and could perhaps ‘not fly in the presence of dogs’ , but not one saying I could ‘fly safely.’ Similarly someone may be allergic to peanuts but I wouldn’t expect anyone to carry a certificate stating they could safely be in the presence of peanuts (peanut factories excepted.???)
I was just on a flight where they announced that one of the passengers had claimed a severe peanut allergy. Their first announcement was that they weren’t going to serve peanuts on the flight. Their second announcement was they asked everyone on the flight to not open or eat anything they brought with them that might have peanuts. The third announcement was they were cancelling all snack service, including beverages, out of caution. So, we couldn’t even have water, because of one person’s supposed peanut allergy that is who knows how severe.