Visual Studio for Windows is mostly C++, plus a lot of other things. Heck, there's JavaScript in there, too.

It used to, they moved to WPF with Visual Studio 2010, now it's mostly WPF with some legacy parts that are still in C++

AFAIK the editor (shell) is WPF (C#?) but the internals are still mostly C++/Win32.

The shell still has a lot of C++ in it, yes (but also a lot of C#).

But the shell is a relatively small part of the entire IDE - the bulk of it is the extensions that actually provide support for various languages and technologies. Those also have some C++, either as legacy code, or because the team just preferred it, but by now C# is definitely the majority of it, and it keeps trending in that direction.

A lot of it can be very non-obvious and defying common sense. For example, if you take VS 2013 (pre-Roslyn), the C++ project system was written entirely in C#, while the C# project system was written mostly in C++.

Even in the .NET Core world, the managed runtime is written in C++, the native runtime (for .NET Native) is written in C#.

Seems plausible, I don't know enough details to argue this.