Though I fully agree on every point, I worry that IRC has fallen more and more into disuse and finding candidates who use it is going to be more and more difficult over time.
But, I think what you're trying to hire are skilled software developers. It's apparently been left as an exercise for the reader to demonstrate any correlation between these two skills.
Filter does not mean barrier.
Kids these days can't use IRC, don't know that gopher still exists, never used usenet...
Being a software developer, or admin, or even technical writter, and never even trying to learn how the Internet looked like before everything was turned to Javascript...How sad..
It's like never riding the bike...