My respect for Linus does not extend to his opinions on programing languages. I haven't found any of Linus's objections to C++ compelling. I strongly believe that a C++ kernel developed with the same care as Linux would be strictly more robust. There are simply more and better tools for constraints and engineered safety in C++.
Besides arguments about portability, C's only "advantage" vs C++ is a lack of features. This isn't an advantage for a serious engineering project, or really any project with code review.
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)
Lack of features can be a good thing. There will be no “language feature anxiety” for the people who is writing code. The code is easier to comprehend for new people who just joined an ongoing project, because there are no non-obvious execution flows. And C code can be compiled so much faster than C++ due to the language being simpler.
I usually spend the compilation time staring at code, only to realize, somewhere in the middle, that I have to abort the build because I must make another change in a large and complicated template in header file that is included almost everywhere. At times, I do miss C, especially its classical variant, where you often didn't even have to include anything and instead could just write the declaration of something you needed - right next to where you need it...
You can still use forward declarations. You can even use them in C++. But you have to be sure to get the signature exactly right (one of my favorite passages from “Design & Evolution of C++” involves Stroustrup’s discovery that a lot of programmers were cavalier about their forward declarations, e.g., using “...” as in any declaration where it would be syntactically valid).
Besides arguments about portability, C's only "advantage" vs C++ is a lack of features. This isn't an advantage for a serious engineering project, or really any project with code review.