Wasn't the first cut at windows me written in c#, maybe we can start there. The servo browser on redox isn't even completely free of c I think.
I think you're thinking of Windows Longhorn (https://en.wikipedia.org/wiki/Development_of_Windows_Vista#M...). They used managed code and ended up partially scrapping it to restart development on top of Windows Server 2003.
Windows ME was just Win98SE in an ugly sweater. Mostly C/C++ with some assembly peppered in.
Windows ME was the worst Windows though.
Vista was the worst in my book. And Vista was actually the real turning point when C# started to infect Windows.
Servo relies on firefox's JS runtime spidermonkey, written in C++.
Moreover, a significant fraction of JS vulnerabilities are logic errors in the JIT, so even if the JIT itself is memory safe, that doesn't make the resulting code free of vulnerabilities.