A cheap VPS could run something like Zulip or Mattermost or phpBB or other ways to communicate. Then that open source community would be in control of their own data.
I pay for Slack, but I'm also part of a number of non-organization communities. They're more like the phpBB communities of the 2000s. Even if I were willing to pay to be a member of those (which I probably am, but not to the tune of $8 each), the organizational overhead of collecting dues from everyone towards Slack fees wouldn't be worth the effort.
