It boggles my mind why not more of these Slack clones implement threaded conversations. It’s such a popular feature and not hard to implement. It could be introduced in a backwards compatible way in order to not break existing clients.
Which Slack clones are you talking about? This comment seems off-topic here, given that threaded conversations are the ONLY thing Zulip implements (way before Slack), making Slack a bad Zulip clone.
Matrix
Matrix has threads.
Discord added threads this year and it’s lovely.
I really like discord's spin on then where threads are actually visible for some limited time. You don't have to hunt for the right message in the channel like in Slack - if a thread was started in the last few days, it's on the list.