What .NET Framework versions support the runtime enhancements for Span<T>?

I understand Span<T> can be used on the .NET Framework as a Nuget package. There have been JIT upgrades to .NET Core to improve performance of Span<T> to nearly standard array performance.

What .NET Framework versions support these runtime enhancements to the performance of Span<T>?

Is .NET framework 4.8 still supported?

NET Framework 4.8 is the latest version of . NET Framework and will continue to be distributed with future releases of Windows. As long as it is installed on a supported version of Windows, . NET Framework 4.8 will continue to also be supported.

Is .NET framework 4.5 still supported?

NET Framework 4.5. 2, 4.6, and 4.6. 1 will reach End of Support on April 26, 2022. After this date, we will no longer provide updates including security fixes or technical support for these versions.

It's unlikely that "fast span" support will appear in a .NET Framework version anytime soon -- and certainly it's not there in 4.7.2 (or earlier versions) and is not in the plans for 4.8 -- see for instance this discussion thread.