We still use email, but not for internal team communication. We have a help article of why Twist is radically different than email https://twist.zendesk.com/hc/en-us/articles/115003654589-Twi...
https://blog.codinghorror.com/discussions-flat-or-threaded/
Flat discussions make much more sense, especially for emails. Gmail and most email clients are focusing on flat discussion threads, which are much easier to follow.