But in recent years, families who asked to preboard or sought information about food served on a flight have been barred from flying or even escorted off planes after taking their seats. The tension and uncertainty have made some passengers reluctant to disclose food allerg ies.

No one tracks in-flight medical emergencies, but they are believed to be relatively uncommon; chest pain and heart attacks are the most common reasons that flights are diverted, according to one study. Allergic reactions comprise fewer than 4 percent of in-flight-medical emergencies.

Two other complaints against American Airlines were dropped by the D.O.T. after the company quietly changed the policy prohibiting preboarding for people with food allergies late last year.

All customers with peanut and tree nut allergies are now allowed to preboard and wipe down surfaces, according to an airline spokeswoman.

The change is not detailed on the airline’s website, which continues to warn that nuts are served onboard and that the airline “can’t accommodate requests to not serve certain food or to provide nut buffer zones” and “can’t guarantee you won’t be exposed to peanuts or other tree nuts during flight.”

Many airlines have policies that accommodate passengers with nut allergies, though disabilities-rights advocates complain that the policies are not consistently enforced.

Delta allows preboarding and refrains from serving peanuts on board if a passenger has an allergy. Southwest quit serving peanuts — its signature snack — in August 2018, substituting pretzels and other snacks.