IRC is irrelevant in today's world. No serious, innovative community is still on IRC.
Slack is not an alternative.
A lot of large open-source projects have Slack communities, often alongside IRC ones. Take Clojure for example, with its 14k-members “Clojurians” Slack project.
Yes. It’s free for non-profit orgs.
I wasn’t clear. It’s free for non-profit orgs. Open-source communities that aren’t registered organizations are by definition not non-profit orgs.
Source: worked on one such team at Amazon in 2017.