If a chicken has acces to a field with grasses, herbs and insects you don’t need to feed layer pellets, crumble or all flock feed. A grain mix added to the things the chickens finds for herself is enough... I have been wortied since observing her, if giving her a diet of whole grains, seeds and forage will suffice, based on the hypothesis that they need the commercial feed.
Of course this doesn’t work if you keep your chickens confined in a coop/run combination. Unless the run is tremendously large.
During winter periods many chickens can’t find enough greens and insects. I dont think its a good idea to feed only grains if you don’t give other food (sprouts/mealworms etc.) or no other food can be found.