VAX/VMS was written in BLISS, it only adopted C after UNIX started to widespread and they needed to cater to competition and their own in-house UNIX implementation, learn history properly.
https://en.wikipedia.org/wiki/BLISS
The even the popular JVM is written in a mix of Java and C++, with plans to port most of the stuff to Java, now that GraalVM has been productised, https://openjdk.java.net/projects/metropolis/
Speaking of which, there are at least two well known version of the JVM written in Java, GraalVM and JikesRVM. Better learn the Java eco-system.
UNIX was written in Assembly for the PDP-7, C only came into play when they ported it to the PDP-11 and UNIX V6 was the first release where most of the code was finally written in C.
IBM i, z/OS or Unisys ClearPath in 2020, have completly different hardware than when they appeared in 1988, 1967 and 1961 respectively, yet PL/S, PL/X and NEWP are still heavily used on them. Looks like portable code to me.
Mac OS, you know the predecessor for macOS, was written in Object Pascal, even though eventually Apple added support for C and C++, which than made C++ with PowerPlant the way to code on the Mac, not C.
BeOS, Symbian were written in C++, not C.
Outside of the kernel space, Windows and OS/2 always favoured C++ and nowadays Windows 10 is a mix of .NET, .NET Native (on UWP) and C++ (kernel supports C++ code since Windows Vista).
NeXT used Objective-C in the kernel, that's right, NeXT drivers were written in Objective-C. Only the BSD/Mach stuff used C.
macOS replaced the Objective-C driver framework with IO Kit, based on Embedded C++ again not C. Nowadays with userspace drivers the C++ framework is called DriverKit in homage to the original Objective-C NeXT framework.
Arduino and ARM mbed are written in C++, not C.
Android uses C only for the Linux kernel, everything else is a mix of Java and C++, and since Project Treble you can even write drivers in Java, just like in Android Things allowed to since version 1.0.
Safe Ada is used alongside C++ on the GenodeOS research project.
Inferno, the last iteration of the hacker beloved Plan 9, uses C on the kernel and the complete userspace makes use of Limbo.
F-Secure, you might have heard of them, has their own bare metal Go implementation for writing firmware and is used in production via the Armory products.
IBM used PL.8 to write a LLVM like compiler toolchain and OS during their RISC research and only pivoted to Aix, because that was what the market wanted RISC for.
Contrary to the cargo cult that Multics was a failure, the OS continued without wihtout Bell Labs and was even assessed to be more secure by DOJ thanks to its use of PL/I instead of C.
There is so much to the world of operating systems than the tunnel vision of UNIX and C.