[pjmlp]

Runtimes and languages go hand-in-hand, a runtime that doesn't support everything that one expects from the standard library breaks compatibility.

Code starts getting full of #ifdefs

Siverlight, .NET Core, WinRT, UWP just to give three reboots.

No support for dynamic APIs, appdomains, IL generation on the fly, reflection APIs done in a different way, ...

Actually C# is not fully backwards compatible, variables declared on foreach statements changed their semantics in C# 5.

https://blogs.msdn.microsoft.com/ericlippert/2009/11/12/clos...

MS-DOS is only supported in 32 bit variants, a species in extinsion.