There is no such thing as “.NET Core 4.8”. There is no such thing as HttpClient’s GitHub repository either - HttpClient was never a separate project and was introduced into the standard library 12.5 years ago as the replacement to then aging WebRequest which had been around since version 1.1.
If you’re interested, .NET’s source code is hosted here: https://github.com/dotnet with the main repositories being runtime, roslyn, fsharp and sdk.
I mean, just the star count on GitHub is enough to show what the developer community thinks.