Please don't use Slack for FOSS projects
It's really difficult for me to take this point seriously if the only proposed alternative is IRC. I understand the benefits of IRC for FOSS and have used it many times in the past, but there's a reason why projects are adopting Slack over IRC. It's easier to use, it deals with on/offline states a lot more gracefully, it allows joining and inviting to rooms much easier, it has a clean and easy to use design and it's available pretty much anywhere using the same interface without having to install a client through their web interface (which is the exact same as their app interface). Along side that, there are hundreds of other minor to major features (File Sharing! Ticketing/build integration! Link Previews! Editing a message after it's sent!!!) that make it much more useful for many use cases than IRC. I think there is definitely room for competition with Slack for FOSS projects, but trying to pretend like IRC is that ultimate solution will ignore the problem completely.