I personally don't feed mine milk/dairy but that would be ok, I just meant the artificial stuff... Sorry for wording that terribly. I looked it up and it said the only issue with giving chickens plain yogurt/milk is its a little hard to digest
Artificial preservatives, food dyes such as Blue 40, Red 5, any color with a number after it. My friend's chickens have had bad experiences with yogurt. Sodium benzoate is a common preservative in yogurt that, with enough, can cause cancer/sickness