It seems that Microsoft is working on yet another version of Windows, and this time it's called Windows 10 Lean. It was spotted in the ISO for the latest Windows 10 Insider Preview, build 17650. Of course, Windows 10 Lean won't be its official name when it launches. It's more likely that this is similar to when Windows 10 Cloud showed up, and that ended up being Windows 10 S.
Spotted by Lucan on Twitter, the OS is slimmed down, with an installation size that's about 2GB smaller than Windows 10 Pro. It's also missing about 50.5k files that are in Pro, including things like wallpapers, the Registry Editor, and MMC management console, and more. Other inbox apps, like the legacy Internet Explorer, Paint, Photos, Maps, Groove Music, Movies & TV, and more, are also gone.
At first glance, this might seem like an early version of Windows Core OS, the more modular version of Windows 10 that would only install components as necessary. It is not. In fact, this slimmed down version of the OS still has Win32 support, something that you probably wouldn't see from a base installation of a more modern version of Windows.
This is more likely to be a slimmed down version of Windows 10 that's made for devices with low storage. Microsoft still lists 16GB as the minimum amount of storage for a 32-bit installation of Windows 10, and 20GB as the minimum for 64-bit. There are still plenty of devices that ship with 32GB, and that still leaves very little room for actual use after Windows is installed.
The Redstone 5 update won't be finalized until this fall, so it will be a while until we see Windows 10 Lean in action. Hopefully, Microsoft will tell us more sooner rather than later.