This is not released, but multiple reputable sources claim it exists (Windows Central, Thurrott, Brad Sims, Betanews, Neowin) and have visualized what it looks like, how it works, claim to have internal Microsoft sources, and have a surprising amount of technical details. This is an AMAZING read and I have posted it here because we should probably brace ourselves for it. It also is the most likely reason why Microsoft is releasing 20H1 builds before the 19H2 builds.
For the project, it sounds like:
Our drivers will still work (or with little modifications and repackaging)
This OS is designed from the heart to run on less powerful devices well, so as to better compete with Chromebooks. Win32 apps should still work, but there will be less “overhead” and “cruft.” This could be quite helpful for running on the Pi…
We may need bigger SD cards (the dual partition update system above)
There is significant evidence we will hear more at BUILD 2019, or at least, later this year.
Zac Bowden at WindowsCentral.com and Brad Sims at Petri.com / Thurrott.com are the most in-the-know about what’s happening. They both claim to have actually seen Windows Core OS running, which is how they were able to make the “reproductions” on Windows Central of what it looks like.
I don’t know if they have any more information than they have already revealed, but you could try getting in touch with them for more information.
I would also suggest you read about the Surface Hub 2X, which we know is running Windows Core OS “Oasis.” Microsoft briefly allowed the press to play with it, and according to Windows Central, it was obviously not Windows 10 as we know it. There were several very clear differences between the 2S (running current Windows 10) and the 2X (supposedly running Core OS Oasis, and had an employee telling him “NO!” whenever he started touching too much).
This article is as-of September 2018. The article in my original post is only a week old and is newer.
Surface Hub 2X running Core OS (this animation is impossible on Windows 10):
As part of this fight against Chromebooks (initally, it will get more powerful over time), I’ve bet you’ve heard about how Microsoft is replacing Microsoft Edge as we know it with a new Chromium-based Microsoft Edge. If you can’t beat 'em, join 'em. Neowin recently leaked early screenshots, and a test build could be released any day to us.
Of course, this won’t run as well on the Pi. But Firefox works better than original Edge anyway.