I said, in the very comment you're replying to, that I do not think the situation would be any different if I knew what the fanon is like.
To repeat: If the recip knows they don't want something that's dirt-common in canon they need to DNW it.
Whether the writer has any familiarity with fanon does not matter. If fanon is all vegan AUs, that does not mean you can't write cannibalism for a blank signup when the canon is Hannibal, even if you know fanon is all vegan AUs. Cannibalism and corpses in Hannibal are opt-out.
Re: Why isn't Yuletide growing?
To repeat: If the recip knows they don't want something that's dirt-common in canon they need to DNW it.
Whether the writer has any familiarity with fanon does not matter. If fanon is all vegan AUs, that does not mean you can't write cannibalism for a blank signup when the canon is Hannibal, even if you know fanon is all vegan AUs. Cannibalism and corpses in Hannibal are opt-out.