Microsoft has decided to stop the new development for .NET Framework.
.NET Framework 4.8 will be the last major version of .NET Framework.
There focus will be on .NET Core. With the .NET Core 3.0 release in September 2019, all new .NET applications should be based on .NET Core.
If we have existing .NET Framework applications that we are maintaining, there is no need to move these applications to .NET Core. Microsoft will continue to both service and support .NET Framework, which includes bug, reliability and security fixes. It will continue to ship with Windows (much of Windows depends on .NET Framework) and we will continue to improve the tooling support for .NET in Visual Studio (Visual Studio is written on .NET Framework).
New applications should be built on .NET Core. .NET Core is where future investments in .NET will happen. Existing applications are safe to remain on .NET Framework which will be supported. Existing applications that want to take advantage of the new features in .NET should consider moving to .NET Core. They have plan into the future, that will be bringing in even more capabilities to the platform. Check out Microsoft plans here.