[phendrenad2]

For the kernel, or for the user-mode applications? C++ features aren't very useful for kernels. Presumably the author is more familiar with C and "C with classes" C++ and someone can rewrite the user-mode apps in modern C++ later.

[gk1256]

Both. SerenityOS, includeOS, Managarm used Modern C++ for Kernel. For example, Managarm used coroutines and Templates also RAII stuff like lock_guard in it's kernel.

[ncmncm]

It is foolish to assert that "C++ features aren't very useful for kernels". C++ features are useful for programs, particularly big programs. Kernels are big programs.