Ugh. I have an anecdote in which I was brought in by a great niche technical contracting firm, as a developer who was also supposed to repair a client relationship, after a previous developer quit. I learned that the problem on client site was not quite what was expected (they already knew the client's manager didn't want contractors). It turned out that a significant part the problem on the ground was simply that the contracting company's on-site manager was bombarding the contractors with instant messages they were required to monitor. There were a few other things (including no onboarding/documentation for client's unique SCM and build environment, some strong personal styles, and a factor that was doing the opposite of what it was supposed to do), but the frustrating message interruptions alone might've made the morale difference that led to the person quitting.
In lieu of Slack, one lightweight pattern that often works is something like:
* Email for things you should see, possibly urgently (ideally with simple filters/actions, for when/how to alert/expose different messages, which the individual can tune over time, to their needs).
* Lightweight chat (maybe IRC/XMPP/etc.) for things you don't have to see (such as questions you don't want to interrupt anyone over, but they can answer if they're available enough to look at chat; and perhaps watercooler chatter for personalizing everyone, especially when you have remote workers). Be careful that this doesn't turn into fear-of-missing-out compulsion or an operational must-see -- emphasize that turning it into must-see is counter to the goal of the medium, becoming a burden for people.
* Agreed-upon simple in-repo/wiki/other for capturing information that people need to see in the future. Try to make this lightweight enough that it's not a burden to capture this information in a way that's accessible/usable to people when they need it.
The organization can evolve this over time, but the above is not a bad start.
A possible side benefit of rolling a lightweight solution on your own is that you don't need to trust a third-party with your company-confidential data (nor inside 'analytics' of your people and company), nor deal with possible goal misalignment with the third-party. (Of course, you can decide any third-party is worthwhile, and sys-admin is usually not what you want to be spending time on in most startups, but you have many options.)