I think Rocket Chat started development after the Slack craze and tried to fill the niche of being an open source version of the latter. Since I last used it, it seems like a good alternative for on-premise, open source Slack-like, still.

Now that we have Matrix, I'm not sure where it's place is but I'm sure some orgs are fine using it for internal communications without having to pay Slack for your history/premium features?