On one large free software project, we adopted Mattermost (an open source / self-hosted equivalent of Slack). We went from ~12 active users and ~50 lesser active users, to ~ 100 active and ~ 500 lesser active users. I agree that more folks should consider moving away from IRC.

The sign-up process for Mattermost is easy. It's intuitive, cross-platform, mobile-friendly, etc. While we geeks are used to IRC, onboarding people to IRC is no different than any other product: the smallest glitch, weird UI, confusing elements will make people drop off pretty quickly. I have always had to sit down with someone to show them how to connect to IRC. We would waste time at code sprints showing people how to connect.

With Mattermost, we have unlimited logs (so we can cross-ref/permalink conversations in the issue trackers), threaded conversations (great for when the channel is noizy), emoji reactions (friendlier atmosphere), etc.

However, I agree with the article: Slack is inappropriate for open source communities. Mattermost and Rocket Chat are good alternatives. Between my IRC habits and what I can do with Mattermost, I find Slack very frustrating (and I don't use it anymore). Then again, as much as I like IRC, it's not an alternative either.