Thanks for your message.
We use the .NetStandard 2.0 library so that it also works in the Simulator, which is currently made in WPF. The Simulator is a powerful tool that lets you debug your OpenSilver application using the full featured .NET Framework debugging tools of Visual Studio.
OpenSilver relies on Microsoft Blazor, which relies on AspNet core. We plan to stay in sync with the technology used by Blazor.
We plan to move to .Net 5.0 when available.