My experience with these communities is that they are very close to non-functional, for several reasons. I use paid Slack, to pretty good effect, every day at work.
First of all, they have to rely heavily on hacks to store information long-term because of the 10,000 message limit.
Second, even with fairly diligent use of Google Docs for important stuff, tons of info is still lost. I've got a slack where I have DMs with four different people starred. But for two of the four, I have no idea why, because I don't recognize the names and the entire conversation history is gone.
Third, where the message limit is less of an issue, and low barrier to entry is important -- ones with a lower activity rate, and where the community is not super close-knit -- tend to suffer from constant challenges to the group's norms. People drift in and out, wanting to argue again and again about how it's OK for them to use exclusively male pronouns all the time, or how it's OK for them to discuss irrelevant topics. This takes up a ton of the message history, and lowers the information density.
Like it or not, IRC, with a web client, and a publicly available archive that is searchable, is far more functional for this situation. Paid Slack is a pretty good tool for work communication, but the free version, for now, doesn't translate well to open communities.