My own post was taken down before, so I'm pretty sure some form of censorship exists. I don't know if it's enforced by mods or implemented otherwise, but that's a minor point.
We go out of our way not to kill stories and comments that are critical of YC or YC-funded startups. When such posts break the HN guidelines, we may penalize them, but always less than we would if the story were about something else. That's a pretty big deal around here—it's literally the first rule of HN moderation.
Beyond that, we'd need to know what post you're talking about.
If you'd like to provide a link, we can look into it.
I'm sure you can see that the post linking to the (simplifying) anti-YC blog being accidentally killed by spam filter can raise questions about whether it was really an accident. I'm not saying it was not - I'm just saying the bayesian probability of this being an accident doesn't look that great to me.
Lots of software affects both story and comment rankings.
"Poison" is a rather tendentious way to describe that. HN's goal is high quality in all things, to the extent it's possible. That doesn't just happen by default; systems are necessary, and it gets harder as numbers grow.
there was a decidedly sexist article overnight (1AM-ish PST) which I (apparently among many others) flagged... and it was gone within a couple of minutes!
But negative comments tend to get downvotes, (avoid gratuitous negativity) so people posting negative stuff should try to support it.