Windows 10x was an edition of Windows without all of the old components in it. No more old File Explorer, no Internet Explorer, no Control Panel, and no Win32 support. Instead, it relied on UWP and PWA implementations. Microsoft planned to re-support Win32s by running it in containers separate from the operating system so that “rot” never occurs. Moreover, 10x also included a serious upgrade to the way Windows installed updates (read: faster, more stable; upgrades will take as long as regular updates), there have been major performance gains, especially for low-end devices, and perhaps most importantly: it was The entire UX is regular, partly due to the removal of Win32 support, and partly due to an increased focus on design consistency. It also has a brand new shell.
Windows 10X is now shelved, but what the article here claims “won’t come” is not what Microsoft said. Yes, the brand name “10X” will likely never be used (but that wasn’t the plan even when it was still actively working out to my knowledge), but the OS is still being developed in-house. New versions of 10X are still being built within Microsoft.
But what’s even more important now is that a lot of that work gets carried over to Windows 10 (which very likely hasn’t been “10” for a long time, by the way). The Sun Valley project is all part of that. Sun Valley is in the process of bringing UX from Windows 10X to Windows 10, optimized for desktops, and updating legacy user interfaces to be more in line with Fluent Design. By the way, there’s something that not a lot of people here at Tweakers seem to understand: Sun Valley isn’t a new design language, it’s a project for applying Fluent Design (and dark mode, etc.) consistently across Windows. But Sun Valley, as far as I know, is mostly UX focused. There’s more in 10x more than that, and that too sooner or later will carry over.
There is a lot of WCOS already in Windows 10. Windows 10 does not support WCOS, of course, but it is a Windows NT-based operating system and will remain so, so parts of it are also shared with legacy OS variants like desktop and server. HoloLens 2 and Xbox are already on WCOS (HoloLens 2 is a bit more than Xbox, of course).