I want to note that Gitter is currently backed by Matrix, and their web client will be replaced by a variant of Element once feature parity is reached. https://element.io/blog/gitter-is-joining-element/
Most of the development effort has been going towards Element. https://github.com/vector-im/element-web/issues?q=sort%3Aupdated-desc+Gitter
Also, matrix is supposed to get zulip-style threads one day. https://news.ycombinator.com/item?id=31132819
Threads are already in beta on some Element clients.
Yes, but they are planning to support both style of thread.
I really like zulip’s threads, but I never had the chance to use it much.