I used to give the in-town chickens an occasional pizza because my spare kid worked at Domino's on the closing shift and would bring home burnt or mis-made pizzas.
Too much fat and salt isn't good for them, but a little bit of pizza from time to time is unlikely to do them any harm -- especially if they are free-fed their regular, balanced feed and have access to plenty of other goodies such as forage, vegetables, weeds, etc.
It was never a lot and they never suffered any ill effects. It was always hilarious to see one of the hens snatch off a slice of pepperoni and take off for a game of chicken football with the rest of the flock chasing her instead of pulling off their own pepperoni.
Foraging in human garbage is, after all, probably how chickens ended up being domesticated to begin with.
