[gamegod]

Mattermost, IRC, etc.

[merpnderp]

IRC is great, but I've never thought of it as protecting my privacy. It's definitely public and almost always logged by a myriad of bots if not the server.

[icedchai]

You can run your own server.

[paxys]

How are either of these more private than Slack? Whoever hosts the server can see whatever is on it.