A lot of people, including me, dislike threading in Slack. It breaks the algorithm for reading new messages. I use it when teammates do, and acknowledge it's useful sometimes, but I would prefer not to have that feature.
Discord also does not have threads. I've heard some complaints about it, but not many.
Threads in Zulip seem to change it into a different kind of app. I don't see how the Slack workspaces that I've used heavily would work with it. There are about 15 channels in my sidebar. Making them nested would turn it into 75, assuming 5 threads per channel. Perhaps there wouldn't need to be as many top-level channels with Zulip threads, but it makes it into a whole different paradigm. They are also missing Slack-style threads. I don't think it would be good to adopt it.
Discord also does not have threads. I've heard some complaints about it, but not many.
Threads in Zulip seem to change it into a different kind of app. I don't see how the Slack workspaces that I've used heavily would work with it. There are about 15 channels in my sidebar. Making them nested would turn it into 75, assuming 5 threads per channel. Perhaps there wouldn't need to be as many top-level channels with Zulip threads, but it makes it into a whole different paradigm. They are also missing Slack-style threads. I don't think it would be good to adopt it.