Use WinRT API in netstandard2.0 library referenced by .NET 5.0 with IsOSPlatform check

We are in the process of updating our netcoreapp3.1 libraries and programs to net5.0. These libraries are referencing one netstandard2.0 library which uses WinRT if supported via a NuGet reference to Microsoft.Windows.SDK.Contracts.
This is checked via RuntimeInformation.IsOSPlatform(OSPlatform.Windows). If I now use this library from net5.0 and execute the part using WinRT I get the following exception (executed on Win 10 20H2 with .NET 5.0.2):

System.PlatformNotSupportedException: Operation is not supported on this platform. (0x80131539)

If I read up on the breaking changes for net5.0 I get that this package is no longer needed/supported and to use WinRT you should target e.g. net5.0-windows10.0.17763.0. But I would like to keep the library platform-agnostic as netstandard2.0 and keep using the IsOSPlatform-switch, as other parts of the library are also used on Linux or using net471.

How can I make this work with net5.0 as we did with netcoreapp3.1?