My personal experience is that the people willing to answer the questions on slack are also the ones who take those answers and put them in documentation so they don't have to keep answering the same questions.
My experience is also that those who don't bother to be responsive to questions aren't pained enough to write the documentation. Further, due to the blindness of expertise, they document the wrong things.
It's precisely the pain of interruption that leads to empathy and good documentation. My team, as one who sources multiple teams, does a significant amount of work in documentation precisely because we know the pain points.
I had a pain point recently when some behavior changed that broke a consumer. I could track to the change, but I couldn't tell why because this person didn't make a meaningful git commit. They didn't create a changelog that explained neither the technical changes nor the product rationale. So, off to slack, and waiting because someone was "too busy" to do any of this.
I have an idea that teams should have a "slack on call" person who rotates daily to interface with other teams. This means that one person is impacted every couple of days. It sort-of works in practice.