I started to work on an independent SAML2 client/server library that would target classic .NET Framework. The source code is available on GitHub, the repo's name is OldMusicBox.Saml2. I decided to release it under MIT license.
The long-time goal is to cover most SAML2 features and target .NET Core.
The short-time goal is to have basic features quickly so that it's possible to use this in other projects. In particular, based on feedback I get from people, I believe the community needs an independent Profil Zaufany client library. This one, when it's ready, will be released under more strict license (so that I could control where and how it's used). The plan is to make it available in few weeks and somewhere in future add the support of the Krajowy Węzeł Identyfikacji Elektronicznej.