For people looking for an alternative to switch to, we’ve been using Quill [0], which is pretty much a more slick, clean Zulip.

It has a great approach to threads, where you can make them mandatory or optional on a per-channel basis (even when optional they have names and are more useful than Slack threads).

They also support adding external parties to channels/threads by email or SMS and even more interestingly, their model is such that you can direct message people in other Quill organizations if you know their details.

[0]: https://quill.chat