I somehow doubt it, most of the APIs (with the notable exception of some of the reflection APIs) are at parity and the language is the same.
This is more like a major library dropping support for a legacy runtime going forward.
What reflection APIs are you talking about? Do you mean things like AppDomain and remoting?
Yes, at least when I last checked, that stuff wasn't present.
When porting some reflection heavy code to .net standard I encountered several breaking changes. They were all easily fixed by using GetTypeInfo(), which is where a lot of reflection functionality seems to have moved. I remember reading that it was for performance improvements.
What version of netstandard?
I just checked the project.json from that library and it appears to have been 1.6.0. It was a while back.