It's mostly manual. Readers often email us cases that need attention, though, and that's super helpful when the top subthread is a generic black hole. If you see one, and have a minute to alert us at [email protected], we'd be grateful! It's probably the single biggest thing moderation can do to improve thread quality, and we can't read all the threads.
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
edit: that is to say, I wouldn't expect user-driven signals to automatically partition or downweight subthreads, I was more curious if they are useful in existing moderation actions and how they might extrapolate.