IRC and mailing lists are still the most important communication channels for a lot of projects (see: kernel.org and similar projects). It haven't changed for 30+ years and I hope it won't change in the future.

Not sure I'd call it primary for all of them, but it's been a supremely useful, and speedy, support resource whenever I've needed it. Being able to just blindly /join #name_of_project and land somewhere useful (even if unofficial) is like magic.