Related from 4 months ago "Ask HN: What’s a good laptop for software development at around $2k?" https://news.ycombinator.com/item?id=31094361 First comment-thread is about Apple but once collapsed the next 10 are non-Apple recommendations.
Were you never curious about what the little [-] did? Where's your sense of adventure! I rib you, but at the same time I understand and your idea has merit.
I might have used it at some time, but must have forgot it since then. I now find it super useful to be able to collapse long threads, to see what others have posted :D
Which means that, when the parent poster started using this site, these Javascript-only thread collapsing buttons didn't exist (I took a quick look at archive.org to confirm). Since these buttons are so subtle, it is plausible that the parent poster didn't notice when they appeared.