While I was maintaining Linkedin's desktop feed experience, I also helped add threaded likes and comments to the desktop feed. Other folks were adding it to the new native Android/iOS applications and the new mobile-web experience.
Threaded likes and comments allowed users to like a comment and comment on a comment and surfaced various new updates related to those actions.
This was a fun project because it required a lot of thought into making a comment more like an update, which itself was already complex due to its recursive nature (updates can contain other updates when someone likes/comments/reshares an update).
I wrote APIs in Scala and modified our existing vanilla js frontend to support this.