Certain parts of Microsoft Learn are permissive, most .NET/C# documentation is Creative Commons Attribution:

* Conceptual docs: https://github.com/dotnet/docs

* BCL: https://github.com/dotnet/dotnet-api-docs

* ASP.NET Core: https://github.com/dotnet/AspNetCore.Docs

* WinForms/WPF: https://github.com/dotnet/docs-desktop

The C# language specification is unfortunately a bit fuzzier, but the conceptual docs above include what most people want: https://github.com/dotnet/csharplang/discussions/4855

The updated unified C# language specification is CC, but it's still catching up to modern C#: https://github.com/dotnet/csharpstandard