So … you will get a wide answer on this. And all of it will be antidotal/based on each person’s experience. To my knowledge there is no actual toxicity studies with backyard chickens on much of anything(exception being moldy chicken feed). There are a lot of extrapolation from other species like dog and cats to chickens.
I can only tell you personally my experience, which is they will eat what they like and avoid stuff they don’t. I’ve seen mine eat many things that have been mentioned as toxic or they ignore it. I’ve had toxic weeds grow in their pen - datura and morning glories, can’t keep those things out! - and none have died. The chooks that I have had died, died of something else, though I suppose a toxic substance couldn’t be ruled out 100%. The only thing I will avoid is cooked bones because of the splintering and digestion issues.
But of course, there’s no chance of possible toxicity if they don’t get it in the first place! Just don’t get super anxious if they get a hold of something that might be possibly toxic - chances are they’ll be just fine!
EDIT: actually to amend a little glancing at the links people put up - I won’t give anything with artificial sugars in it. I wouldn’t give alcohol - that’s never come up anyway!
Sorry, EDIT again: I’d only give “everything” to adult chickens, not growing chicks!