All commercial chickens in my country have a vegan diet with supplements to stay healthy.
What you like to give your chickens is up to you, but it’s not unhealthy to give vegan feed to chickens. Or humans with the right supplements. Many people who hate animal suffering or don’t want to contribute to global warning and the use of pesticides /herbicides choose to eat vegan.
My chickens eat organic vegan feed + now and then: some mealworms / some cheese crust /yoghurt + whatever they find free ranging. And they live happy long lives.
As already stated, only the feed would be vegan, not their entire diet.
It's fine to give them veggies or fruits as a treat too, that's also vegan friendly. But they shouldn't have to survive on only that...
I have ducks not chickens but what I say still applies, I'm vegan and don't buy my girls meats or anything- but I do give them insects and encourage foraging as that is a natural behaviour I would never deny them. I also give them oyster shells which are not vegan.... but without that how are they going to regulate their calcium intake?? I could never prevent my birds from foraging for insects and worms, that's half their entertainment as ducks and a nice part of their diet.
And when the opportunity arises to expand their diet, I take it. In a way that is not me directly causing any harm to the planet or to any animal of agriculture. For example I currently have a bag of shrimp that would otherwise have ended up in the bin, which I hope they'll enjoy. Even if it makes me uncomfortable. Because their happiness and health come before my lifestyle.
The people originally mentioned here seem to be badly neglecting their chickens diet due to their lifestyle as vegans, and that isn't okay.
Heck, I believe dogs can survive on a plant based diet, based on the theories from countless sources. But I wouldn't put mine on one until I see hard evidence supported by numerous vets and studies. I hope that day comes, and it may not! There sure as heck isn't hard evidence supporting vegan diets for chickens yet.