I'll come as close as I can. The question was whether photons are assumed in most posts about Bell's Theorem & entanglement.
Bell's Theorem itself used spin 1/2 electrons as the base example. But because photons are much easier to prepare, and because the resultant tests also can control for the locality assumption as well, photons are almost always used in experiments. I would say that most posts assume the discussion is about photons and spin. Technically, entanglement can be seen with other observables besides spin.