People made up standards for what is or isn't desirable in a breed. In reality, any "breed" can be bred to be any pattern. A breed is more of silhouette/shape that produces a consistent size/shape/color of offspring which is done through a form of inbreeding called line breeding. Line breeding produces genetically similar offspring over time that look uniform and have similar disease resistance and susceptibility. The longer a rooster is used to breed his line/daughters or sons to their mothers, the weaker the (blood)line becomes. Typically it is a max of 7 generations before new blood is needed.
Overall, yes, a non-bearded Mille Fleur d'Uccle is against the American SOP (American Bantam Association), and these birds won't place or earn trophies. A more common example you'll see on this forum is how American Silkies are super plush versus the standard held by most of the world which calls for our "pet quality" silkies.
In my opinion, bird SOPs are a silly concept, but are indeed a goal that some people work toward. I don't agree with the intentional culling of birds who don't fit imaginary guidelines we created as I feel even a chicken with a "squirrel tail" deserves automatic death/"culling" due to someone's interpretation of being undesirable--but this is a common practice in breeding to a Standard of Perfection (SOP). Some breeders do rehome, but others don't to "protect" their genetic stock. Ethics vary widely when it comes to livestock.