Even though the version of reference assemblies is set here it is not respected and all arcade repos on version 6 build with <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0">
Roslyn will need to work around this in custom targets. If it is intentional that customers build against version 1.0.0 then the property in DefaultVersions.props should be modified or deleted.