[benparsons]

Self-hosted, decentralised solutions such as Matrix (https://matrix.org) are available, and allow you to keep control of your own access and data.

[NoGravitas]

I keep hearing people complain about Matrix's interface (Riot), but the little I've used Slack, it looks exactly the same except for the color scheme.

[benparsons]

Riot is currently undergoing a lot of UX work, the results look really promising. Take a look in January for an update.

Disclaimer: I work on Matrix (should have made that clear in the parent comment too)