My favorite slack alternative is Zulip; IMO it's got a much superior model; it's as much a real-time forum as chat. It makes threads first class citizens rather than the ugly hack they added in slack.
And it's open source, so it's decentralized in that many people host their own. It's not federated though.
I think this is a matter of preference. Microsoft Teams does threads as first class citizens, and to me it's overly busy. I'd much rather a chat tool work like IRC, and in my mind Slack is simply IRC with more features.
Creating a subject for a thread is optional in Microsoft Teams rather than required as it is in Zulip. This very minor difference makes a HUGE difference in how the UI works.
And it's open source, so it's decentralized in that many people host their own. It's not federated though.