I've edited my previous post and added a link to the docs which mention 3.22 as requirement.
The Mingw CI with GTK 3.8 passes only by accident in this regard because this PR's changes are `#ifdef`ed for Linux :).
About the rest of the version confusion, I'm lost as well. I don't know how serious is the Glade file requirement is. It seems even Ubuntu 18.04 had already GTK 3.22, so it's probably not a big issue and we could maybe make Meson and Autotools require GTK 3.22 as well.