If you're leaving over the 90-day history limit
Microsoft Teams — its free tier has no message history cap, which is the single clearest advantage over Slack free. Zulip is free and open source with better threading than either, and self-hostable if you want to own it outright.
If you're leaving because of the interruptions
Twist is built async by design so nobody needs to be online simultaneously — it's the most direct fix for ping culture and deserves more attention than it gets. Zulip's topic model also cuts noise substantially.
If it's a community rather than a company
Discord, without much argument. Voice that feels casual rather than scheduled is a genuine advantage.
If you're four people who mostly need to coordinate
Then the honest answer may be that you don't need a chat app at all. Most small-team Slack traffic is status — who's working on what, when are we free, is this done. Fuidzy's shared timelines answer all three without a single channel, and each person controls exactly what others can see.
Free history → Teams or Zulip. Fewer pings → Twist. Community → Discord. Just coordination → a shared timeline, not a chat app.