Unfortunately, the feature is no longer in development but Turner said Microsoft is still working on providing a tabs feature for the command line of Windows.
ARS Technica writes Microsoft cancelled Sets because it was too difficult to implement:
Sets was an extremely complicated feature. Initially, Microsoft planned only to use it with new programs written using the Universal Windows Platform (UWP). That's because UWP programs are generally well-behaved. Programs built using the old Win32 API can do all sorts of strange things, such as drawing menus into their titlebars (as Visual Studio does), having ribbon controls merged into the title bars (as Office does), or even putting tabs into their titlebars (as Chrome does). UWP applications, in contrast, do have some control over their titlebars, but it's much more controlled and consistent.
The Shell-provided tab experience is no more, but adding tabs is high on our to do list.— Rich Turner (@richturn_ms) April 20, 2019