Not OP, but I would never try to build an open-source community on Slack.
I'm not going to pay $6/month for each person who wants to sign up to check it out, and I'm not willing to lose message history (quicker and quicker as popularity increases). If someone comes in with an issue it sure would be nice to be able to refer back to the conversation when somebody else had a similar issue two months ago.
100% - the slack pricing model doesn't work with OSS communities.
Also it's a lot of overhead to join a new slack group for a random comment. It makes sense for the discussion to happen in the same environment as the code.
I'm not going to pay $6/month for each person who wants to sign up to check it out, and I'm not willing to lose message history (quicker and quicker as popularity increases). If someone comes in with an issue it sure would be nice to be able to refer back to the conversation when somebody else had a similar issue two months ago.