I'd contact Spiritsmam directly and ask her what sh eknows. She runs a rescue where they save these dogs. I think it's very infrequent and obviously they are just now discovering that there MIGHT be some kind of rare genetic component to it.
She's the only person I've heard of that enconters these dogs. But it could be that breeders are culling them if they notice it. It does seem pretty obviously, but I have no idea what these dogs are like when they are first born.
I know of around 30 affected GSD's and hear of about 5 - 10 more per year. Yes, breeders often cull affected pups. Puppies are born "normal", becoming straight legged when they are about 4-8 weeks old. Some dogs we have pedigrees for, others are dumped in shelters so we have no idea.
We have had the final necropsy report on my dog and the pathologists have never seen or heard of anything like it before. It may be an inherited motor neuron disease, but much more research will be needed before any conclusions can be drawn.