A user marked it offtopic, which downweighted it. A few longstanding HN users have the ability to do this, as part of an experiment we've been running. I've written about it in a few places:

https://news.ycombinator.com/item?id=31228950

https://news.ycombinator.com/item?id=31227642

https://news.ycombinator.com/item?id=31227584

https://news.ycombinator.com/item?id=31117569

The downweighting is intended for when overly generic or offtopic subthreads are stuck at the top of a page, choking out more interesting or on-topic conversation.

(Unfortunately it caused some trouble a while back because one such downweighted subthread was critical of a YC startup, and a much more important rule is that we moderate HN less when YC or YC startups are the topic. But it was a straightforward mistake—the user who marked the thread generic just didn't realize that the topic was YC related.)

Since it's an experiment, we want to keep an eye on it. Currently I have the software send an email each time a downweight is applied, so we can review them as part of going through the regular HN inbox. When I saw this one, it was such a perfect example that I came to the thread and auto-collapsed it as well. So that's its current status: downweighted and auto-collapsed.

Btw, basically none of these situations get created intentionally. They're a tragedy-of-the-commons thing where each person does what they do innocently but it all adds up to something suboptimal. That's pretty much what moderation exists for: to jiggle the system when it gets stuck in one of its failure modes.