There’s always Riot https://about.riot.im/ which is a nice and fully open source app so you could set that up on-premise too. :)
Unfortunately, I suspect Slack use at my company grew largely out of IT's unwillingness to spin up a web-facing IRC solution, and their unwillingness to expose internal services to the web. The latter is very convenient for sales or on-call employees. As a result of that security posture, we instead share tons of company secrets on a 3rd party service. It's kind of a weird local maxima.
Now that everyone "knows" what slack is, I wonder if it's time to blow some dust off of Apache (née Google) wave...
(I guess the other caveat is: it's a 3rd party hosted application; an internet outage or service outage could take it down, and it's potentially vulnerable to massive information leak in a way that your company isn't.)