If you have just a few users, Rocket is very easy to install and has always worked well on a $5 DO instance. My preference is for Zulip, as it's easier to track multiple discussions, but I don't have an incentive to switch now that my Rocket instance is running.

I have no experience with more than a few users. If you want a hosted solution, I have seen reports that Zulip works well with a large number of users.

There's Matrix and Riot. it's still a bit rough on the edges but overall it's very cool projects.

He asked about slack, not Whatsapp. ;)

Riot has inline Markdown, so I'd argue it's actually superior to Slack for serious work.

I've used Riot as a Slack replacement and setup notification bots. Its effective for the task and I could use the IRC bridge for a similar function.

Only reason I moved over to Discord was too many chat apps and the hobby users were all on Discord anyway.

Riot is more slack-like than whatsapp-like. We use it at work instead of Slack now.

Discord is great. Pitched to gamers, but could easily function in a work setting.

Mattermost.

Came here to say this. Mattermost runs really well on a small-ish box, self-contained, has never gone down, and we've been able to grow organically to hundreds of unique users a day all inside the firewall. Pretty much a no-brainer for me.