Long answer: My colleagues don't want to so I use it mostly for forums. Some programming languages have chat channels and I have heard that for open source dev it is still somewhat common but I suspect it is declining. It's a pity because I think Slack is stupid. Slack does not have the transparency of IRC and frankly the fact that you can share files and use fonts and indents are not selling points for me.
EDIT: Slack is stupid for me. To each their own I guess?
EDIT2: Just to make it clear, judging from other answers a lot of programmers use IRC every day. But not all of my colleagues are programmers. Some fall in the chasm between "data science" and programming and for such people IRC is unfortunately unfamiliar.