Tim Hortons used to do this in the early 2000s specifically for peanut (and maybe some other) allergies. In retrospect I'm sure it wasn't fun for my parents to explain to 10 year old me that I suddenly wasn't able to have donuts anymore (My mum used to take me and my brother to Tims as a treat every now and then, after they did this we switched to McDonalds (who are low-key amazing about food allergies). I haven't ordered anything from Tims ever since)
Depends. For some allergies you just have to do your research and it's fine. McDonalds of all places is very good about keeping nuts and peanuts in sealed bags and away from everything else (to be fair they basically are used in nothing but the sundaes). Obviously substitutions are a different thing, but for some allergies fast food isn't that difficult if you just know the place you're going to (most dietary restriction info is available in the apps anyway these days).
If people are going to show up and demand things are catered to them that's one thing, but there seems to be a vibe in this thread that people with food allergies are somehow "unreasonable" for a thing we have no control over. Like obviously I am not going to go to a Thai restaurant and demand they don't put peanuts in things. I'm not an idiot. But like, I think the idea every restaurant should just ban people with food allergies is cruel. Just because I can't eat one specific food I shouldn't be able to go to a restaurant? Don't lump those of us who know how to be responsible about it in with people who aren't.