Will objects that go beyond the cosmological event horizon eventually vanish from sight?

Sources that say that the light is redshifted into the indefinite future are talking about classical cosmology and not considering quantum effects.

Quantum mechanically, as you say, the galaxy emits only finitely many photons before crossing the horizon and so there should be a last photon – although I should qualify that in a couple of ways. First, unless you know all future measurement outcomes, the disappearance still takes forever in the sense that you can never be sure that you've seen the last photon; another one could arrive at an arbitrarily late time. Second, even if you do know all future measurement outcomes, the horizon emits Hawking/Unruh radiation, and because of boson indistinguishability I'm not sure that there's any way even in principle to unambiguously identify the last galaxy photon amidst that noise.