Holy moly, first thread summary I've seen, very cool! Is this an automatic thing now? (assuming url match I suppose)
Dang is a moderator, and he does it when he thinks it's useful. I suspect he does it "by hand" but, like me, has written scripts to help.
Given that your account was created 12 years ago I'm a little surprised that this is the first thread summary you've seen.
You'd certainly be welcome to resume! I think including the date + number of comments is the important bit, plus not pointing to threads with zero comments - but IIRC you had both of those features.
Edit: one fun fact – it's surprisingly difficult to post links to past threads without coming off as somehow reproaching the latest submitter for posting a duplicate. In reality, HN allows reposts after about a year (and this is in https://news.ycombinator.com/newsfaq.html), but many people don't know that and interpret the list of past threads as an implicit criticism, which is a shame.
I've tried out a bunch of different wordings trying to minimize that misunderstanding. The simplest one seems to be the most effective - I just say "Related". Somehow the word "related" sounds less like a criticism. It also has the nice property of being inclusive, so for example it's fine to include other articles on the same topic.
One of these years, I still want to build software support for collecting related URLs and related past threads into HN's official UI. Then we can all build up sets of related things collaboratively. That will hopefully make HN more interesting.
You could also take all the posts and related links, toss them in a big adjacency matrix and, oh I dunno, rank the relatedness of pages.
Dumb jokes aside, these do add a lot of extra structure to the dataset and people might think up something interesting to do with that.