Yeah, any of the the cultures/fermented things from feed to salt-free kraut (or other veggies) to buttermilk, kefir, yoghurt, etc are excellent for chickens. Like granny hatchet said, these foods will add back good gut bacteria, not a bad idea for general bird health and essential if they've ever had antibiotics.
Benefits are a long list, but some include: firmer, less stinky poops, less feed required since the bird is getting more nutrition out of their food, healthier birds(more resistant to diseases), possibly grow and/or molt more quickly, possibly lay more eggs. Also with fermented dairy products like buttermilk, it's a good source of calcium for layers. It's best not to give unfermented milk to chickens because they can't well digest lactose. Fermenting the milk breaks down most if not all the lactose. Mind you, the benefits will be most noticeable if you also ferment their feed in addition to the buttermilk.
We had one girl that laid very soft shells. She LOVED milk kefir or plain yoghurt and the occasional fermented feed. After a few months of this, she started laying much firmer shells, quite normal, actually. My hunch is she was having digestive problems and the fermented things improved her health.
I think I read recently that the famous Label Rouge chickens in France (similar to our certified organic) are finished on buttermilk before slaughter and supposedly makes their meat tastier or more tender. I've heard of similar methods on cattle and goat being finished on grain that has been soaked in apple cider vinegar. It makes sense as whenever I marinate meat in some sort of acid (usu lemon juice or ACV) it makes a big difference in flavor and tenderness. Why not do it from the inside out?
