It is reasonable to raise the point that building FOSS software while using a closed piece of software as a core tool bears consideration. However, I am 25 and largely missed the boat on irc. I decided to start using it ~1 year ago. It is hard to use, but we will look at that in a minute. Often, we assume people have as clear an idea of what we are talking about as we do. That is often not the case. Let's explore IRC…
It's worth mentioning that Zulip ( https://zulip.org/ ) is a new open source option that has basically all the features IRC/Slack does. So using open source tools for open source development doesn't have to mean dealing with IRC's limitations. (I'm one of the Zulip maintainers; happy to answer any questions about it).
Less importantly: What would you say its biggest missing feature is vs Slack? And conversely what does it bring to the table that Slack can't touch yet?