The issue for me is that in discussions like this we conflate toxicity with sub-optimal nutrition.
As
@U_Stormcrow describes with his pocket full of Quarters analogy, bread can displace higher value foods.
On the other hand, bread is not toxic.
So a ‘never give bread’ is really not a sufficiently nuanced warning to give for example to a new chicken keeper.
Soak your bread in milk and suddenly you have something that is protein and calcium rich.
I don’t eat that much bread so my chickens don’t either. At this time of year they eat mainly whatever they find for themselves. But if it were winter with snow on the ground, and my chicken feed got spoiled or eaten by raccoons, I don’t think I would hesitate to feed my flock bread and milk until I could get a replacement bag of feed.