Announced as in "it was described as such in the blog post that announced the Vouch feature": https://blog.ycombinator.com/two-hn-announcements/

That's of course not to say that it's widely known, many details of HN are not well-documented, only mentioned occasionally.

Exactly. It shouldn't be this hard to learn about all the features of the HN commenting system, especially given how minimalist it is.