Egg shells are a recycling of calcium the natural way. I have been doing this with my flock for over 20+ years. Some people bake, and crush their egg shells. I just drop on ground in run area,, and step on with shoe to crush. Never a problem. Chickens find as they come across them...
I would not be too worried about them overindulging in the oyster shells. Whatever does not get used up, for eggshell interior production,,,, will pass thru the chickens' digestive track.
It is a possibility,, that the sickness they went thru may have changed their needs for calcium...