In November 2017, Microsoft announced Sets, a new idea for a tabbed interface in Windows 10, which would allow you to group apps in tabs in the same way that a browser arranges webpages. The firm started testing the feature in the early previews for Windows 10 version 1803, and later removed it. Sets came back in Redstone 5 testing, and was once again removed.
Not a word about Sets has been spoken since until yesterday. Senior PM Rich Turner tweeted that the "Shell-provided tab experience is no more", and also that "adding tabs is high on our to do list".
Obviously, there's some ambiguity in that tweet, and it would be nice if Microsoft actually tried to communicate more effectively. But since there wasn't even any word about Sets in Windows 10 1903 testing, it's probably safe to say that Sets in its previous form is dead. After all, it relied on Microsoft's Edge browser, and Edge is currently undergoing a Chromium overhaul.
One theory is that Microsoft is planning to bring back Sets via another method. Also, the conversation on Twitter was about tabbed pages in the Windows Console, something that has been the most up-voted feedback since 2014, long before Sets was ever a thing. It's possible that Turner was just talking about adding tabs to the Console.
After the ambiguous message this weekend, it's possible that Microsoft might actually try to communicate what's going on sometime this week. You probably shouldn't hold your breath though.