Sets was being testing by Windows Insiders in RS5 preview builds. But with the release of the Insider Preview Build 17704, Microsoft has pulled all traces of the tabbed interface. However, it’s not going away forever.
Microsoft said that they’ve made “Sets offline to continue making it great” based on the feedback received. There is a lot to be done including design improvements and better integrate Sets with MS Office and Edge to improve workflow.
Redmond has promised Sets will return in future insider builds, and eventually, in some RTM build. But it’s less likely that the ambitious feature that would change how we use Windows would land on our machines this year.
For the time being, third-party developers have already come up with alternatives to Sets. One of them is ‘Groupy,’ which bring the tabbed interface to Windows 10, Windows 8.1, and Windows 7.