Earlier quoted context omitted.
Here's a few examples I know of: https://news.ycombinator.com/item?id=9015734 https://news.ycombinator.com/item?id=8938409
The first link is a low-quality comment / ad hominem, the second link is off-topic / ad hominem. Both of those were already against the rules.
Sharp readers may point out that the HN guidelines have always excluded those things. That's true. But it's still enough of a problem in HN threads that this is a clarification worth making. We tried it out last year when we released special guidelines for Show HNs. It worked well there, so we're extending it to the whole site.