People talk about Discord on here for chat rooms a lot. Is it open in some way that Slack is not? Looking at their website it seems like it's basically... just Slack.
I've been musing, this morning, about Twitter:Mastodon::Slack:???
@dconley Slack is also Slack but with voice chat, now.
Someone mentioned Matrix.org, which seems like what I had in mind, at least roughly.