No such thing in the ADA. Likely your employer asks you not to ask paying passengers what is wrong with them, and frankly, given the disdain posters display of passengers that potentially "get away with it" I can understand why they would issue guidance against flight attendants from inquiring.

Imagine the passenger experience of being asked by the GA then FA then purser "what's wrong with them?" in a suspicious fashion.

That being said just eliminate the whole spectacle of special handling IMO. If you can't walk the length of a plane in a reasonable time, you probably shouldn't fly.