I'm trying to find the source, but I read the other day (I think from a Twitter exec on Bluesky) that all of the features launched since Musk bought Twitter were being worked on long before Musk bought Twitter. Which is as you'd expect, at least for the majority. These things don't happen overnight, or even in the space of a few months.

Also, bookmarks have been there for years. As have subscriptions, and Community Notes.