Span<T> only requires .NET Standard 1.0, so it came out for Core and Framework at the same time [1].

.NET Framework only has the portable implementation though. The fast implementation requires support in the runtime, and Framework may never support fast Span<T> [2].

[1] https://www.nuget.org/packages/System.Memory/4.4.0-preview1-... [2] https://github.com/dotnet/coreclr/issues/18280#issuecomment-...