> the large majority of the technical candidates I've been interviewing have no visible trace on GitHub (either because they worked on closed source, or because they are out of school without not a lot of personal drive to work on OSS).
Or because they work extensively on OSS projects that don't use GitHub.
Actually no (not in my case, I mean! YMMV). Something I can tell with certainty is that in my recruitement channels for the last 2 years, people either do OSS on GitHub, or are not doing OSS at all.
That might be true in your recruitment channels, but in general it's a problematic assumption. Many prominent Open Source projects don't use GitHub; in particular, that applies to those large enough and/or old enough to have and maintain their own infrastructure. I've run into that problematic assumption a few times, and it seems worth calling attention to.
However, many prominent Open Source projects that don't use GitHub have mirrors on GitHub, and as long as you star them, they appear in your GitHub profile if you contributed to them using an email that is registered to your GitHub account.
Or because they work extensively on OSS projects that don't use GitHub.