[pjmlp]

Using C compilers written in C++, yeah really difficult to escape C.

Windows has been migrating into C++ since Windows Vista, when VC++ introduced kernel support. Its C universal runtime is written in C++ with extern "C" entry points. Ah and then there is COM/WinRT, also mostly about C++ and .NET, even if the ABI is compatible with C (for masochists).

macOS uses C++ for drivers, a kernel without drivers isn't of much use.

Speaking of which, in Android all drivers written using Project Treble (required as of Android 8) are either C++ or Java based, and now Rust is also into the picture. The only C is the "classical" Linux upstream stuff.