The problem with that is, some things (like cannibalism and violence against children) can be a specific “ick” for some people who are otherwise comfortable with content you might expect to see at whatever general rating you want to aim for. And if I rely on the players to say “I’m fine with R18 generally, but please don’t have my character unknowingly eat the flesh of human children” there’s a good chance they won’t think of that, even if it would be very upsetting to them if it happened. And, if I ask them about that specifically, they’re going to expect it to come up. But, if I present a brief questionnaire about how comfortable they are with a number of subjects, including cannibalism, violence against children, starvation, sexual conduct, sexual violenice/assault, graphic descriptions of violence, unwilling bodily transformation, etc., I can get a sense of if I should just cut that content from the adventure, without spoiling the moment if nobody objects to the first two.