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.
Imagine making a scoring system for domains that that takes into account flags and votes on comments that include said domain. Also, your post had a word written in ALL CAPS, arguably two. You can see other comments linking to the same domain at around the same time [1] that aren't marked as spam.
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.
Nobody found them inflammatory when they sat in the public HN source code for years, but once they appeared in an accidental screenshot, they became outrageous!
We're happy to answer good-faith questions about HN moderation, but this seems like trying to stir up drama.
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!
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.