I built the protobuf-net 2.4.0 project for .Net Standard 2.0 but the portability-analyzer tool shows only 92% of compatibility because of the below details.
Questions are:
It is, confusingly, impossible to talk about compatibility with .NET Standard, unless you mean "lowest common denominator", which: most people don't. What matters is what you actually run it on. You mention .NET Framework and .NET Core - and it should.work just fine there. If you were using it on Unity or Xamarin (mobile etc), the answer is more complicated.