To be honest, blue styrofoam has been an important part of the diet of our new pullets last year, and although we tried to put it out of reach, they somehow always managed to find more of it. Some days, they ate so much of the stuff that their poop had a weird bluish color. We realized they had an addiction, so we scheduled a meeting with the whole flock to confront the young ones. Obviously, it took them a while to get over the denial. But they finally gave in and accepted to go into rehab. It was a long and grueling process, but they finally came out of it. Well, most of them, I guess. Sadly, we recently caught a few of them still indulging in their craving for blue styrofoam. We suspect some of them are keeping a secret stash somewhere. But we now have an insider working for us in the flock. It is costing us a bundle in corn but, sooner or later, the wisegirls will be busted.
So you see, if the stuff was toxic, all of our young pullets would be long dead. And trust me, they are all mighty healthy.
Oyster shell is not nearly as hard as granite, but it's still a good deal harder than styrofoam so it can and does work as grit.
Chickens have some sort of weird fascination with styrofoam as they quite often eat it. Can't say that I've ever seen it hurt them, but it's not doing them any good either so it's best to keep it away from them as much as you can.