Much as the title says, I really miss the Usenet days where I could contribute to a bulletin board-style forum on hyper-specific subjects.
If I remember my computing history correctly, Google ended up acquiring and eating Usenet, becoming Google Groups. I'm not sure if this is dead yet.
Arguably, Reddit fills some of this niche, but Usenet was tech-focused, generally quite professional and frankly didn't have the same clientele as Reddit does.
HN is topic-focused, rather than subject-focused.
Would be very interested to see if there's any Usenet-style project that's still alive.