Back in 2018, it was reported that Microsoft is building a new, modern version of Windows 10 for PCs, codenamed Polaris. This would be under the Windows Core OS umbrella which also included Andromeda as the operating system for foldable mobile devices. It was rumored that this flavor of the operating system will strip much of the legacy code and compatibility issues that the OS is currently encumbered with. While Microsoft canceled both Polaris and Andromeda in 2018, and tapped Windows 10X as their replacement, a build of the former has now leaked online.
Windows Central reports that an old build of Polaris - the modular flavor for traditional PCs - has leaked online, but unfortunately, there's not much to be excited about. The leak is a very early build from 2018 and contains no shell or apps. This essentially means that it has no GUI either and is just the bare OS package.
While there's not much that can be messed around with in the build, some interesting tidbits are that it is an RS3 RTM build and is compatible with ARM32 architecture. This means that it can be booted up on the age-old Surface RT.
Polaris was supposed to be a modular version of Windows, offering better flexibility, power management, and performance. However, it is important to emphasize that the OS is no longer in development, and has been superseded by Windows 10X, which is slated to arrive in spring later this year. As this is a leaked build of the software and could contain undisclosed issues, we would advise against trying to tinker around with it on your primary machine.
Source: Windows Central