I think there is a health case to be made for variety in diet. I don't think it's just a case of anthropomorphism. Chickens apparently require 8 amino acids to construct a complete protein. There are however many more amino acids and various possible combinations of these acids that will also provide a complete protein. Then there is also the point regarding the omnivorous nature of chickens, yet many are fed on a vegetarian diet.They would have to know chicken nutrition...not just general nutrition. And feed IS proper, balanced, correct nutrition for the animal and purpose for which it was made. It’s just that humans tend to apply human emotions to animals and then think that the animal appreciates the variety, etc. I feed my chickens treats on occasion, however I rely on feed as the backbone of their nutrition. Of course, I can easily source feed. Most people would not know what chickens needed or what was in the trash items they were feeding their chickens.
A proper comparison over a life of a chicken fed on selected scraps compared to a commercial feed would make an interesting study.