It was called “Flocks”. It even had transient/temporal flocks based on geo check-in and sound-fingerprint flocks for movies, tv shows, and events. We implemented the Shazam audio signature algo from a research paper we found.

The thesis was: it’s more natural for people to have conversations when there’s a shared context.

It became a widely known project within the company back then, but ultimately, like most things at Twitter, never shipped.

Maybe Google can try replicating Reddit in YouTube - it would just be serving text instead of video; discovery would be via their recommendation algorithm, i.e. no subreddits. Frankly, they can kind of already do this, you can post community messages on YouTube. They just need to tweak the algorithm to recommend such messages.

edit: great idea though it functions pretty much exactly as you described, get into product if you're not yet!