When I was working in fast food we got a lot of people telling us that they were deathly allergic to onions. If all of their claims were true then every man woman and child in my city of 300,000 would have to be eating their at least once a week. It's a major disruption because if someone claims an allergy you have to do a special mini prep just for that order to avoid cross contamination. After a while we collectively just started treating them like regular no onion orders. I'd be shocked if most places didn't do the same.