https://www.bleepingcomputer.com/news/microsoft/microsoft-pulls-windows-10-sets-from-redstone-5-builds/

Last year, Microsoft announced a new feature called "Sets" for Windows 10 that would enable applications to run in tabs in UWP apps and Win32 programs. After a long wait, the feature was implemented in Windows 10 Insider preview builds, but the feedback has led Microsoft to remove Sets from its latest Windows 10 preview build 17704, which is being released today.

Microsoft isn’t planning to ship browser-like tab-based navigation feature, dubbed Sets, with the next major Windows 10 update (Redstone 5) later this year. In a blog post, the software maker has announced that they are pulling public development of the feature, which means you won't be able to test it in preview builds either.