IIRC, Gentoo still uses CVS as well (which is arguably more fundamental to the project since it's source-based). The original creator of the OS now works on Funtoo, which is based on Gentoo but uses git.
It was an interesting migration project to follow seeing how the portage tree is huge.
Somebody corrected this (Gentoo VC software) later, but NetBSD does also still use CVS. Repo conversions are indeed big, interesting projects. I've closely-witnessed/been-peripherally-involved in a few, and there were some interesting papers written about various conversions years ago[0], when both git and mercurial were new and iirc monotone still had interest.
[0] both Mozilla and Sun were publishing their analysis and growing pains