FYI HackerWeb has collapsible threads:

https://cheeaun.github.io/hackerweb/#/item/10698009

It's designed for mobile web, but just zoom in and it should be fine

I wrote a bookmarklet for this a year ago and have since been using it every day: http://www.sagargv.com/proj/flathn/

If you search, there are lots of good plugins and bookmarklets to do the job. Yeah, it would be nice to have it built-in, but people have been asking for it for years - I don't think it's likely to happen soon.

EDIT: Apparently posting really long strings without spaces breaks HN's layout now. Hunh.

Here's my bookmarklet for that:

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