Some things still seem to be Windows specific. If I want offline documentation (of the sort one can find at /usr/share/javadoc/java or /usr/share/doc/rust/html by installing the correct packages), every place I look tell me how to enable offline help in Visual Studio (for instance, https://learn.microsoft.com/en-us/teamblog/offline-book-refr...). Someone here told me last time that there's a way to download whole sections of the MSDN documentation as PDF, which helps, but it's not the same thing.
Off-line documentation being kind of a pain sounds like a very minor thing.
Other than that, and other than the obviously Windows-specific GUI and system management libraries, the rest of C#/.NET is pretty much fully multi-platform, maybe except a few obscure things.
