.NET Framework will be supported for much longer than that. It is used in many internal Windows components including MMC snap-ins, ADFS, and other Microsoft technologies like Exchange and Sharepoint.
Or maybe it will not be supported as in "we won't provide support for any applications needing .NET Framework, but they may or may not still work"?
Or to phrase it another way: Microsoft themselves failed to migrate off .NET Framework to .NET Core for the same reasons their customers are also stuck.
PowerShell Core has been available for years but still is not included with Windows.
Similarly, even new web UI components such as Admin Center are written using .NET Framework.
> Microsoft themselves failed to migrate off .NET Framework to .NET Core for the same reasons their customers are also stuck.
I'd wager it is more a question of priorities. I bet Microsoft itself sees Windows Server as legacy tech as they want everyone to move to the(ir) cloud.
Partially it is also a organisational problem. Everything you ship as a Windows component incurs a debt, which is exactly why .NET Framework wasn't able to evolve further.
Or maybe it will not be supported as in "we won't provide support for any applications needing .NET Framework, but they may or may not still work"?