They can get calcium from bugs, certain plants, frogs, mice and other creepy crawlies with bones, and, if you live in limestone country, from the rocks they use as grit. If the egg shells are OK, you don't need to offer any extra, whether oyster shell or egg shells. I offer oyster shell on the side and it is hardly ever touched. They hardly ever eat the egg shells out of the compost heap. They are obviously getting enough elsewhere. A lot of the time, when I have a mixed aged flock, I feed Starter or Grower to the entire flock, so they are not getting any extra calcium from Layer. They eat a bit more of the oyster shell then, but it still lasts a very long time.
I don't see any harm in offering them egg shells or oyster shells on the side so they have a choice of whether to eat it or not. Personally I don't mix it with their feed. Why force them to eat something they don't need and their body has to work to get rid of the excess? But then, my egg shells are fine. Many people do it differently.
Growing up on the farm many decades ago, my parents never gave the chickens oyster shell or any commercial feed, anying with extra calcium in it. The chickens free ranged all the time and found all their own food. Those egg shells were fine. But I don't keep my chickens under those same conditions in the same environment, so I offer oyster shell on the side.
I suggest offering oyster shell (or egg shells) on the side. They are pretty inexpensive, If they need them, they are available. If they don't need them, they will last a long, long time and be really inexpensive.