It's not that it's not allowed, it's more if the CMs find they ducks, they are supposed to toss them. But my experience has been, most CMs love the ducks and love to find them themselves.
I'm in a duck hiding group and people are still hiding and finding tons on every cruise. Disney has not come out and said--people, ducks aren't allowed. They would easily be able to stop it by confiscating ducks found in luggage the way they do steamers, extension cords and large scissors and all the other crap found on the table of shame found when you exist the ship.
From what I understand, DCL cannot come out and say--we support duck hiding! Leave your ducks all over our ships! Because if someone gets hurt, a kid chokes on a duck (some are really tiny) or if someone hides an inappropriate duck and someone gets offended by a duck, DCL will be at fault.