Long grass clippings are a total no. They can cause crop impaction, which can eventually lead to death.
I do feed grass clippings, but very short ones (set your mower to a high mowing height if the grass is long, then go over it a few times lowering the height each time, so you're not getting a long clipping). I would use tender succulent clippings, too - not ones from a dried out lawn that might form an indigestible mass in the crop.
I freerange two flocks, but alternate who goes out on what days - the indoor flock gets a little bag of clippings. Works great for chicken entertainment, health and egg quality.
I even feed dried clippings in the winter. I collect them in the summer and dry them fast so they are very green and sweet smelling when dried (and they are 100% dry so no mold forms...), store them in grain bags, and feed out in the winter as a treat with some grain. No dried clipping is longer that 1" long when I feed it.