I've been on HN for a long time (this is not my first account even) but I had completely forgotten about pagination on HN. Even when you talked about it I only get a vague feeling that I've seen it ever. Probably I have seen it but still it feels like the sort of memory that one has of something that didn't actually happen but which you have a "memory" of because someone told you it happened.

I guess I usually simply stop scrolling before I get to the very bottom of HN.

Just to be clear though, are we talking about a "next" link at the bottom of the page, or Reddit-style "continue" links for subtrees of comments beyond a certain depth?

Same. I think that they enable it on a case-by-case basis as load gets high. Also, the "Who's hiring?" threads do not typically spawn active subthreads, so that may make them more cacheable/easier to load.