FWIW I've never known anyone using it as a low-key way to suggest FTT. I don't know every domme in the world, though.

When it's been requested they always wanted the panties worn, but not wet or dirty. That said, I am very explicit about not offering FTT so perhaps the clients after dirty panties are simply not contacting me under the (correct) assumption that I wouldn't offer gagging with dirty panties.