I explained it here: https://www.reddit.com/r/programming/comments/5xlmc6/a_new_h...
Only thanks to this kind of work one can prove it is possible.
https://www.infoq.com/presentations/csharp-systems-programmi...
There are quite a few OS projects written in system programming languages with GC, all the way back to Mesa/Cedar at Xerox PARC.
If you want to have an idea, check the Project Oberon book.
http://people.inf.ethz.ch/wirth/ProjectOberon/index.html
This is the 2013 re-edition, it was originally published in 1992.
No one would though of writing an OS in C++ in the 90's, it was too slow and everyone knew that microkernels could only be possibly written in C.
The "everyone" who knew these things about C++ was a small set of vocal people largely from the Unix and C worlds. Other people who were at the time developing Windows NT device drivers in C++, to pick one example, knew that a lot of the received, and loudly recirculated, wisdom about systems programming and C++ was just bunkum.
What I was trying to convey with that comment was that C++ was looked down for systems programming, it required Symbian, BeOS, OS X IO Kit to change that mentality.
Even Windows that you mention, which always was a more welcoming place for C++ tooling, Microsoft only started to officially support it with Visual C++ 2012 when the /kernel flag was introduced.
So even that microkernel could have been eventually written in System C#, if there was enough management support to keep developing Midori and improving the language to the point C++ wouldn't be needed.
Instead the project was canceled and since in IT, seeing is believing, there will be idea for many, that there wasn't any other way to accomplish it.
I love the way Alan Kay and others like him think, it is not what the technology give us today, but what it can give us tomorrow in a few years time.
Many hobby kernels (as mentioned in these comments) have used C#, as well as bleeding-edge research kernels e.g. Microsoft's Singularity project: https://en.wikipedia.org/wiki/Singularity_(operating_system)
https://www.microsoft.com/en-us/research/publication/safe-to...
It's legacy will be in their IronClad Apps. Look into those if you want to see how far they've gone. Also, Microsoft Research & HLL vs Nix and C in security are a gap that's ever widening. ;)