I find that Slack creates worse "accidental" documentation than some other forms of communication. Prior to my team's adoption of slack, major decisions/designs were usually documented in an email at some point,
because they needed to be broadcast. If you can find that email then you have a fairly good self-contained resource for understanding whatever it was about.
Slack is easy and it's broadcast, so some people use it preferentially whenever they need to broadcast. However, the form that it takes is lines of text barfed out and interleaved with unrelated lines of text. There's no summary metadata (like subject lines), there's no organization except time, people put very little thought into what they type so it's at best rough draft form. It's hard to search, and even worse, your team or organization may be hip but cheap, and just get the free version that disappears your message history making search moot.
In short, we used to get better documentation for free, now we get none. Sometimes, old emails can be immensely useful in trying to figure out why something was done, or recovering old understanding and history.