There are multiple responses in Which is the latest version of ASP.NET? - .NET - The SitePoint Forums. Note that the question does not ask about MVC but the answers are for MVC.
One response says that
ASP.Net MVC 5.2.3 is latest version before they went to
ASP.Net Core MVC. See asp.net mvc 5 - What is the latest ASP .NET MVC version? - Stack Overflow; there does seem to be a lot of confusion. That thread sure says that the Core version replaces the non-Core version. However see Choose between ASP.NET 4.x and ASP.NET Core | Microsoft Docs. That article sure seems to indicate they each have advantages and disadvantages. The non-Core
ASP.Net does not support Web Forms so it seems unlikely it will go away soon.
Does anyone know if there is an official Microsoft response to the question of whether they will continue to improve the non-Core version?
ASP.Net websites are strange and different from classic web sites where you can just create a HTML file and the server sends the file to the browser. Or in the case of server-side pages such as PHP and ASPX pages the page is first processed by a CGI style processor.