Yes, they have stopped laying. I think they are eating chick starter now as I read it was higher in protein. Interesting that you bring up oyster shell. I add it to food or put it out separately along with grit and can tell they never touch the stuff. I’m relentless and keep putting it out but no avail. I feel as if I’m wasting money but given the cost I still make it available.They do need protein to grow new feathers--but if they have stopped laying eggs, they can use the same protein that would normally make eggs to make feathers now.
One of the easiest ways to give them extra protein is to feed chick starter for a while. You want it unmedicated, and you should put out a dish of oyster shell to provide calcium, because laying hens need more calcium than young chicks.
Chick starter is usually easy to get, not too expensive, tastes good to chickens, has an appropriate balance of all the nutrients chickens need, and typically has more protein than most other chicken foods. (Read the label to be sure.)