I would guess this one, but I don't think any usernames carried over. Except maybe the one I can't recall right now (startup related to streaming video over .gif back in late 90s iirc).
But what happened was that Joel freaked a bit out over some of the drama on the Business of Software forums and shuttered them, in spite of them being very popular and having a huge community - another one of those weird decisions. In an alternate timeline, they would have been HN.
-it was getting less and less popular
-the software that ran it was getting creaky
Also I suppose it was marketing for FogBugz. When Trello and StackOverflow took off, it wasn't really needed anymore.
Joel was a brilliant writer. I miss his insight.
I think it's successor peaked in popularity way after that but started to drop off as there was no obvious way to get new users. The old method of people checking their HTTP logs for referrers to see who was linking to their content and then joining the conversation stopped being a thing.
I think BOS eventually died a natural death.