The Sets feature is a tabbed-windows experience that lets you group together different apps on your desktop. Think of it like having different tabs open in your browser, but for apps and File Explorer. It’s a nice way to organize and access a set of apps and makes it much easier to navigate between them and improve your workflow.
Details on why it was removed and when it will come back have been vague. Microsoft made the announcement about Sets in today’s blog post about preview build 17704: I’ll have to admit, in my testing, the Sets feature had its share of issues – especially with Office 365 apps. Also, it wasn’t working with a lot of third-party apps, and even some Microsoft UWP apps. It has come a long way, though, and it was looking promising. But I have to applaud Microsoft for wanting to get it right before officially releasing it to the public. Unlike Windows Timeline, Sets is a big change for the overall Windows experience and will require a bit of a learning curve. If you want a tabbed-windows experience on Windows 10 now, you can get Groupy from Stardock. While Groupy does a good job of offering a similar experience, it isn’t as sophisticated as where Microsoft is going with Sets. If you would like to learn more about what Sets will be and see it in action, check out the following video from Microsoft: