In particular, a lot of preparatory refactoring towards supporting this has been merged over the last year, and we have a contributor planning to focus on this project over the summer.
https://github.com/zulip/zulip/issues/12309 is probably the right issue to follow for now. My plan for today included writing a much more technical version of that issue with a specific implementation plan, before someone submitted Zulip to Hacker News :).
I'm working on a similar threaded messages project called Booklet (https://booklet.community), and I think good email summaries are the key to the product. It's hard to do, but there are some cool data opportunities to improve relevancy.
"You can do that by changing your code like this: [code] Let me know when you have the results."
"Okay, but I'm going to be busy for the next few days. Will post here when it's done."
{Three days later}: "Your suggestion worked! Let me know how to proceed from here."
You can either keep Zulip open all the time - which makes me cringe - or you can go without any form of notification. Then a week later you decide to check back and see the message, but they forgot to @ you.