4.8x ships and still gets security updates
ASP.NET Web Forms sites are especially stuck, and on top of that I have customers that have developed Web Forms sites in VB.NET! Luckily there are some good bulk-conversion tools available now, but still, there is no smooth upgrade path for a lot of popular systems.
Similarly, Windows Communication Foundation, Workflow Foundation, and a bunch of other popular libraries or frameworks are dead in the water. SAP Crystal Reports is surprisingly common, but doesn't even have an official NuGet package!
But if you have developed a .NET Framework app that you want to see living into the future a decade or more from today, you need to know that maintaining it is going to be increasingly painful as time goes on. You are locked to an old version of the C# language, and you absolutely can't count on third-party dependencies to stay supported. At some point MS might well decide to drop tooling for .NET Framework development in new releases of Visual Studio.