Downvoting is often used to express extreme disagreement. Ideally it might not be, but there are no real guidelines on how to use downvoting, and no good mechanism to enforce them if they existed.
It seems to me like the problem isn't that there are certain things you aren't allowed to say, but that there certain things that a large group of people aren't going to even bother taking seriously. It could be because they've seen the argument before and consider it both obviously wrong and inflammatory, or it could be because they've been brainwashed / self-deluded. Regardless, that is a problem with humanity, not the hacker news community.
It seems to me like the problem isn't that there are certain things you aren't allowed to say, but that there certain things that a large group of people aren't going to even bother taking seriously. It could be because they've seen the argument before and consider it both obviously wrong and inflammatory, or it could be because they've been brainwashed / self-deluded. Regardless, that is a problem with humanity, not the hacker news community.