I agree with most of what other people have said.
An old type farm, which produced all a families food, would have had so many more natural food sources then 'most' back yard chicken keepers.
For example, when ever they would have butchered a sheep, cow or pig, the chickens would have tidied it up. They would have had so many more insects, greens, and old fashioned cultivars of grains.
One of my chicken flocks just gets a hand full of grower feed each in the morning, and in the evening one small handful each of 50/50 scratch mix and lupins.
I have not seen any different in their health to the others that have grower feed available all the time, and scratch only as treats.
The first flock free ranges all day, the second is kept penned.
The first flock may lay a few less eggs, but its hard to tell because so many get lost after they lay them in odd corners where I don't find them.
I do wonder, in human food, dog food, cat food, horse food, there is increasing concerns about highly processed food, some saying they cause cancer.
No one seems to worry about this in chickens.....I think you need to make your own mind up about that.
As far as I am concerned, I know when ever I can avoid it I don't want my chickens only eating highly processed feed.