And if you're looking for an example of a kernel being developed in C++, look at Zircon [1] (the kernel for Google's Fuchsia project). It's certainly past the "trivial complexity" phase, and doing just fine.
And of course, things like RAII are incredibly useful when managing resources, and templates for encapsulating generic data-structure behavior in an operating system.
So to me (I'm no C/C++/Kernel/etc. expert) it looks like they are writing the kernel mostly in C, with C++ goodies sprinkled in some places, where it's useful (ie: RAII for spinlock guard)
And of course, things like RAII are incredibly useful when managing resources, and templates for encapsulating generic data-structure behavior in an operating system.
[1] https://fuchsia.googlesource.com/zircon