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???