Windows Core OS (WCOS) is Microsoft’s new unified Windows OS in works, and is considered to be the future of Windows. It is supposed to power all kind of devices and new form factors.
Now, it seems that Microsoft is preparing its official apps for the final release with Windows core OS. Sticky Notes may be the latest app to get Windows Core OS support as an image posted on Twitter reveals.
— Aggiornamenti Lumia (@ALumia_Italia) February 21, 2020
In the screenshot the OS name is Windows Core with maximum version as 10.0.19556, which is similar to the latest insider Fast Ring builds.
Some recent leaks had revealed that Microsoft may be planning to separate the Windows Core OS and Features experience for various versions of the Windows CoreOS. It also appeared in a Geekbench listing and in official documentation sometime ago.