U.S. airlines now require that passengers wear masks in order to fly. They’ve been lobbying the federal government to require it, but it seems like compliance on planes is actually better than average compared to many states where it’s a legal requirement.
Passengers who have medical reasons not to wear a mask are exempt from the requirement. Delta is going a step farther than other airlines (as they often do, for better and worse) requiring passengers who want to claim a medical exemption to have a medical consult prior to travel. If you’re faking, you get your money back.