Which features? Mostly data classes, a.k.a. records, and (from C# 8) non-nullable references. Kotlin has had these features for years and it makes things so much easier and cleaner IMHO.
I believe they tried to keep the C# version independently from the framework version, so people can use the new C# version on a old .net framework.
But things seem impossible for some new features, for example async steam just need new feature from core CLR.
So after .C#8, they decide only to support it on .NET core