Yes, this is already a thing in .NET Core 3: https://docs.microsoft.com/en-us/dotnet/core/whats-new/dotne...
It's not clear to me how the .NET 5 feature is different.
It's not clear to me how the .NET 5 feature is different.
If you don't have daily familiarity with these tools, they are absolutely inscrutable.
Most of this is already true because of .NET Standard 2.0, which latest of Mono, Framework, and Core already implement. Large swaths of the extant OSS libraries for .NET have already converted to .NET Standard 2.0. There are mostly only niche use cases left that today that you can't build into .NET Standard libraries and import into programs compiled for any of the extant CLRs.