WinHttpHandler removed from .NET runtime
WinHttpHandler class was removed from the System.Net.Http.dll assembly. It's now available only as an out-of-band (OOB) NuGet package.
In previous .NET versions, the WinHttpHandler class is available as part of the core .NET libraries. Starting in .NET 5, the WinHttpHandler class is only available as a separately installed NuGet package.