In SV they have started to split. In AKC, you'd still show together. If you part your dog's outer hair, you should be able to see a shorter, thicker undercoat. If you don't then I'm pretty sure your dog would just get disqualified in the AKC ring, and if not disqualified then it wouldn't place very high among a bunch of stock coats. A long coat isn't a disqualifying fault, but it is a point fault so all else equal, a shorter haired dog will be more successful in the breed ring.
The reason you can't cut the fur is that some breeders will shave/cut their longer stock coated GSDs in order to make them look like short/medium coated GSDs. They'll cut the hair on the ears and the feet (usually the biggest give away of a long coat) so that their dogs will have a better chance in the breed ring.