We too have deadly nightshade plants pop up every year, and I do try to remove every one as I find them. The chickens free range where they are, and there's never been a problem. I think offering them in quantity to confined chickens would be bad, but given lots of free range choices, all will probably be fine.
Before I knew what they were, I let them grow in the horse pasture, and fortunately the horses never ate them! It's actually a very handsome plant...
Here they pop up anywhere that we disturb the soil, so apparently the seeds live a long time, and only need exposure to grow. At least it's easy to pull out of the ground!
Mary