Type forwarders should exist in an assembly targeting .NET

[AArnott]

Another targetframework assembly should be added to the package for .NET 6 (the oldest supported runtime) that replaces all your shim types with type forwarders so that code that compiled against your package can run in a .NET 6 environment with other code that compiled against the official Rune type.
That is, if your Rune type truly is otherwise binary compatible with the one in .NET.

[osexpert]

I wonder, what is the advantage of using type forwarders? Is it that you don't need to use Condition in PackageReferences like eg. this?
<PackageReference Include="Shim.System.Text.Rune" Version="6.0.2" Condition="'$(TargetFramework)' == 'netstandard2.0'" />

[AArnott]

That can help that case too, yes. But more importantly, it allows for binaries that already built targeting a framework that needed the types from your assembly will be able to be used in a newer runtime where the types come from another assembly.