Untreated grass seed is fine. Yes, expect your chickens to eat a lot of the seed. Yes, because its untreated, expect little of it to sprout. Yes, your chickens may eat the sprouts, too.
More importantly, where are you, and what kind of grass are you considering??? Not all grass is the same. Fescue, for instance, has toxicity issues for many grazing critters - chickens largely avoid it, unless its their only option. Annual rye is a good winter grass, but its a "one and done". Perrenial rye is much more expensive, but my birds ignore it till it goes to seed. Orchard grass is better, tolerates some shade, but may not be suited to your climate (and even then, its of little interest to the birds, generally). Then there's timothy...
You should also consider flax, vetch, alfalfa, clovers, winter peas, and a host of other greens of various classes (cereals/grains, legumes/pulses, grasses, etc) so your birds have nutritional variety, and so as not to deplete your soil with a monoculture.