When we designed IdentityServer4, we wanted to make it easier to extend the core token service with custom protocol endpoints.
So one thing that comes up every now and then is using IdentityServer4 as an identity provider for SharePoint and also older ASP.NET applications using System.IdentityModel (or even WIF) and Katana.
This requires support for WS-Federation – and this is perfectly possible – and actually even easier than it used to be in IdentityServer3. I made the code for it available on github and wrote a little walkthrough of how it works here.
Our documentation now also has a brief description of the underlying mechanism and useful tips for implementing other (custom) protocols. SAML2p anyone???
And last but not least, our friend Scott Brady wrote a detailed walkthrough on using the WS-Federation endpoint with SharePoint. Enjoy!
leastprivilege.com 
This is awesome! WsFed support has kind of hampered adoption of the .NET Core in a lot of regards. Do you happen to know the status of WsFed middleware for external authentication?
Thanks. No – no news.
This is super useful.
Just want to know that if I can configure a Custom Identity provider (written using WIF) to be used as External Provider in IdentityServer
Yes – by using the new WS-Federation handler from Microsoft.