"Sculpt is an open-source general-purpose OS. It combines Genode's microkernel architecture, capability-based security, sandboxed device drivers, and virtual machines in a novel operating system for commodity PC hardware and the PinePhone. Sculpt is used as day-to-day OS by the Genode developers. "
Google's Fuchsia or at least parts of the Zircon kernel use C++. [0]
{ A subset of the C++17 language is used in the Zircon tree. This includes both the kernel and userspace code. C++ is mixed with C (and some assembly) in both places. Some C++ language features are avoided or prohibited. Use of the C++ standard library features is very circumspect.}