@Mary4426 @Anonymous
FWIW
Airbnb prevents its own link to one of my listings from appearing in another listing, so it's not always true that airbnb.com links are allowed. (I don't know whether this would be the case in messaging.)
I have noticed that whether links are hidden in Airbnb messages depends on the current state of the reservation. If the reservation is confirmed, they are not hidden, even in messages where they previously were hidden. And if the reservation is not yet confirmed or has been canceled the links are hidden, even in messages where they were previously not hidden. So in Airbnb messaging hiding appears to be a real-time event based on reservation status. Of course, this is not true for email copies.