> Async streams, indexers and ranges all rely on new framework types that will be part of .NET Standard 2.1. ... .NET Core 3.0 as well as Xamarin, Unity and Mo

> Async streams, indexers and ranges all rely on new framework types that will be part of .NET Standard 2.1. ... .NET Core 3.0 as well as Xamarin, Unity and Mono will all implement .NET Standard 2.1, but .NET Framework 4.8 will not. This means that the types required to use these features won’t be available when you target C# 8.0 to .NET Framework 4.8. We are approaching the point where .NET full framework becomes legacy, deprecated by .NET Core. NetStandard 2.1 things like Span<T> are already in NETCore 2.1 now, but won't be in the next release of fullFramework, .NET Framework 4.8. Which .NET Framework will they arrive in, and when, if ever? That question has not been answered, even with an approximation or roadmap.