Just published a working code of a port of the fido2-net-lib library. The library is great, it works but there's a drawback - it targets .NET 6 (or higher) which means that creating Fido2/Webauthn compatible website in .NET 4.X using the library is impossible.
My ported library, called the Fido2.NetFramework is supposed to fill the gap. I downgraded the code so that it compiles using the old C# 4.8 compiler, I replaced all external references that targets APIs/libraries/frameworks that are not available in .NET 4.8. For example, the Nsec.Cryptography used in the source library, was replaced with BouncyCastle.
The code contains a working demo.
There's still some work to do, in particular, validate the port by porting unit tests and creating extra compatibility tests.