if they free range than scratch is fine, in the summer and spring months when they have plenty of bugs and greens to eat.
In the 'old days' they fed the chickens a lot more than scratch, in the summer they free ranged and in the winter they lived in the barn with deep litter hay that attracted bugs etc for them to eat, essentially making a free range environment in the safe warm indoors.
Besides the scratch that is fed today is not what was fed then, if you open a bag of Purina scratch grains it is almost all corn with barely any oats, millet etc.
I buy grains separate and mix them, so mine do live on only scratch and table scarps.