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Yes and no. Chickens fed only layer feed (after onset of laying of course, feeding chicks layer feed which has too much calcium for them before they are at or close to point of lay can cause kidney damage) and absolutely nothing else will probably be fine without oyster shell or grit. If your chickens are getting any kind of treats in addition to their feed (scratch, table scraps, greens, bugs, etc) then they will need grit so that they can digest them. If you have sandy soil, your chickens may be able to get enough grit on their own without you having to supplement. But many people like to provide grit just in case. Likewise, if your chickens are eating enough treats and/or free ranging in addition to their layer feed it will result in a reduced amount of calcium which could lead to soft or thin egg shells. Inadequate calcium can also cause egg binding issues in laying hens. If the treats your birds are getting are mostly high calcium treats, then they may not need additional calcium. But again, many people would rather make it available to them rather than risk the problems that come with low calcium.
I too use two old rabbit feeders from when I used to raise rabbits way back when. One for oyster shell and one for grit. Works perfectly for me. Before I remembered that I had them, I had also tried just using a small deep dish (they kept tipping it over) and a milk jug tied to the side of the run with a hole cut in the side for access (worked ok, but it was kind of ugly and unstable with the heavy shell and the shell got wet and solidified into a cake).