I have an affectionate, devoted, fifteen-pound tuxedo cat named Milo who loves nothing better than chewing on feathers. He is so good to us I want to indulge him, if I can do that in a healthy way.
Our vet explained that as "complete predators" cats are best nourished by eating their entire prey - fur, feathers, bird feet, skulls, etc. So I believe eating feathers is not inherently bad for him, perhaps even the opposite.
But feathers are full of fungus, mites, viruses, bacteria, and goodness knows what else. Presumably the digestive system can handle this, but we'd like to clean the feathers if possible. Google "sanitize feathers" and you get responses from crafters who aren't going to eat their feathers, so they're not shy about using bleach, soap, borax, hydrogen peroxide, alcohol, etc.
I could buy feathers, but who knows what they use to clean them.
Most feathers are illegal to collect, of course, but there are non-migratory turkeys around here that drop huge feathers that are great for chewing. At the moment we just freeze them for a couple weeks, but I'm not perfectly satisfied with that.
Milo is a neutered male, 100% indoor cat. Boy, does he love eating feathers.