“Please don’t use Slack for FOSS projects” is a compelling case for why you shouldn’t use Slack for Free and Open Source Software projects. Make sure to read the discussion in the comments as well. (By the way, many of the arguments apply to HipChat too).
The suggested alternative is IRC, which I agree with. My only minor disagreement in regards to IRC is using it for companies as well. Companies are much more fragile and sensitive than Open Source community, so it doesn’t work all that well in some places. I think Slack/HipChat work great for company communications, but if you want to have full control over your chat system, then try out Rocket.Chat, which I blogged about earlier this year.