Now that asp.net is end of life, these sad people are looking for ways to migrate their aging asp and asp.net code base to elsewhere. The only supported option is the incompatible new asp.net core on dotNet core - out of the jungle into the jungle.
It would be helpful if you can provide some source for this.
Even Silverlight is not officially dead until 2021 so I'm not sure I'm clear on your terminology (vs. https://support.microsoft.com/help/17455 ).
Just for fun I dug up the "Mainstream Support End Date" for ASP.NET on .NET 4.5.2: 10/9/2018.
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Starting from https://www.asp.net/support
Many parts of ASP.NET are a part of the Microsoft .NET Framework, these include ASP.NET Web Forms, Controls, Modules, Handlers and more. For more information see .NET Platform Support.
Then at https://www.microsoft.com/net/support
The Support Lifecycle information for all versions of .NET Framework can be seen on the Microsoft Product Lifecycle site.
Then at https://support.microsoft.com/en-us/lifecycle/search?alpha=....
For more information, please visit the .NET Framework Support Lifecycle Policy FAQ.
Then at https://support.microsoft.com/help/17455
Support for .NET 4.5.2 follows the lifecycle policy of the parent operating system. It is supported as a Windows component on the latest required operating system update for Windows Vista SP2, Windows 7 SP1, Windows Server 2008 SP2, Windows Server 2008 R2 SP1, Windows 8.1 Update, Windows Server 2012, and Windows Server 2012 R2.
Then at https://support.microsoft.com/en-us/lifecycle/search?alpha=W...
Products Released: Windows Server 2012 R2 Standard
Lifecycle Start Date: 11/25/2013
Mainstream Support End Date: 10/9/2018
Extended Support End Date: 10/10/2023
Service Pack Support End Date: [empty]
Notes: Mainstream and Extended support for this product has been extended to provide customers with the standard lifecycle transition timeline. For up to 6 years of additional support beyond Extended Support, please see the Premium Assurance website.
[disclaimer: Microsoft employee, .NET Foundation staff]