Having just migrated from Zulip to Slack, I find Slack's threading model really bad. Discord is a little better, but still fragmented over time in a busy channel. Zulip has the best implementation I've seen for putting semi-related topics together without getting drowned out.

Surprisingly, Slack's sales seem unaware of Zulip from what I heard from some colleagues so I doubt Slack is going to steal Zulip's (much better) threading model any time soon.