Lots (all?) major frameworks push for dependency injection though, where interfaces are a must, as far as I know (not for DI-the-principle, but DI-as-implemented). ASP.Net Core is a good example. It's not C# at the language level forcing interface-driven development, but frameworks like ASP.Net are so tightly integrated that boundaries blur.
You don't need interfaces for dependency injection, you can inject classes directly. I see interfaces more for separation of concerns (with domain-driven design pushed from all parts in the .NET world at the moment...), but that's not an obligation.
Yup, that's what I meant by "practically." Nobody forces you to use the Microsoft.x nuget packages, but good luck at finding packages that don't rely on Microsoft.DI, or Microsoft.Hosting, or what-have-you.
