I've always considered that fearmongering that started from feed companies. There's this myth that too much protein will make them fat or if you don't use layer feed they'll struggle to produce shells from lack of calcium. I think a lot of it is hogwash. Protein doesn't make any animal fat...it helps build muscle/strength. Let them free range 100% of the time and all they will eat is bugs/worms and vegetation. Soy is an incomplete protein and can disrupt hormones and corn is very low in nutritional density. Soy and corn are very cheap too so that's why these feed companies love to use it.
I give oyster shell on the side for the girls that think they need the extra calcium, but that's it. One of my 2 8 year olds is still laying...in fact she pushed one out today. 5 of my 8 7 year olds are still alive too...knock on wood.