I have hunted hounds for 30 plus years and have seen a handfull of silent hounds, your question was "are plotts the only hounds that are silent?" NO! you could find one every once in a while in all the hound breeds but you are more likely to find a silent Plott or Redbone than you are in the rest of the breeds. I believe that is because Plotts and Redbones back in the day were used more on big game like bears and Mt. Lions. In the coon hunting world all competative hounds Must be registered! and con not be silent. If they are silent they will be disqualified! because your 4 main breeds in the coon hunting world are Treeing Walkers, American Blueticks, English Hound (both Bluetick and Redtick), and the Black & Tan. Its my opinion thier open mouth genes have been more concentrated than perhaps the Plott and Redbone have. This is just MY OPinion after being in the hound world for so long. Im sure there are other opinions out there. Just trying to answer your question best I could.
I have never been disqualified for hunting a silent dog in ukc, akc, or pkc. It's stated in the ukc rules that a dog can not be silent, but there is no way to prove a dog is silent. He might just tree a lot of "lay-ups" if you know what i mean. A lot of your BIG money winners in pkc are silent, 100% independant, and accurate. People breed for this type of dog because they can get off by themself and keeps you out of a calling contest. An open mouth dog has a much harder time getting by them self when you are hunting with a bunch of me too dogs.
