It provides protein but also methionine, which can be harmful. If you give cat food, the quantity should probably be quite limited. See the note at the bottom of our Treats Chart:
There are many options. Any protein is beneficial but animal sourced protein is particularly a good idea, since chickens are omnivores. A lot of people switch to game bird feed during molt. It is higher protein. Note that it does not supply the extra calciium they may need when laying which of course they don't need during a molt, anyway. Some game bird feeds contain fish meal or other animal sourced protein, too. Some people give mealworms for protein.
You also don't want to give a ot of dairy since chickens are lactose intolerant. If you give any, choose forms that have the lactose already processed such as kefir or natural yoghurt. Some people do give milk or whey; probably, it causes a mild diarrhea and perhaps a "stomach upset.:"