There are a couple of protocols that do similar things, but nearly all of them have a major fatal flaw, that is unfortunately very difficult to work around through no fault of their own: mobile push notifications, especially on iOS. An open protocol is useful, but not enough, and that's where Slack hosting and developing everything themselves has an advantage, despite open protocols and tools being what I'd normally want to use. But practicality matters, and so when I last looked at alternatives a year ago, I couldn't find anything that would be a drop-in feature-complete replacement. It's a shame, but I expect something to come up in the future, perhaps on Matrix or other protocols.