We should probably just remove distro-specific instructions altogether and instead state more clearly what is required.
Anyway, that's something for after the holidays.
This is obsolete now, Debain introduced alternatives for it again for whatever reason:
https://tracker.debian.org/news/1407576/accepted-gcc-mingw-w64-252-source-into-unstable/
* Restore the ability to parallel-install POSIX and Win32 thread variants.
Nothing extracted yet.
Debian unstable made mingw posix and mingw win32 non coinstallable, so new method is simply choosing one option instead of using update-alternatives.