Well, off the top of my head, the things folks should NOT give to chickens are popcorn (unless it is unsalted), anything with lots of sugar - a cupcake now and then, without frosting is okay but not several at once.
Chickens are omniverous, which means meat scraps are perfectly fine. Most folks don't put meat scraps into compost, anyway, to prevent predators from being lured to it. Perfect to give to chickens!
My next door neighbors toss all their kitchen scraps over the fence to my flock in a specific Treat Spot. I warned them about salted popcorn... And left-over frosted stuff. They even toss their old flower arrangements to the flock, as I have ducks and geese which relish the blossoms and greenery, even if the chickens don't care for them.
Citrus and avocado skins/seeds go into the compost. Some folks believe citrus affects laying. Avocados are high in something poisonous.
That's about it. Mine don't seem to like onions, but many folks don't feed onion skins to their chickens, anyway, for fear of changing egg flavor.