The demise of Windows 10 Mobile has been speculated for months, fueled even more by Microsoft's Joe Belfiore that new features and hardware would no longer be a focus. So it should come as no surprise that the Windows 10 SDK has removed the official definition of the OS from the latest update.
Within the older versions of the SDK, Windows 10 Mobile had been defined as PRODUCT_MOBILE_CORE. The newest version has that line removed. What that means is that any apps using the new SDK would not be able to verify that a device was using the Windows 10 Mobile OS.
This also suggests that, while Windows 10 Mobile did get a Fall Creators Update in October, it is likely insiders will see no new builds with anything other than just cumulative updates. Windows 10 Mobile never did see a Redstone 3 or Redstone 4 build, as the Fall Creators Update came straight from the feature2 branch.
The move fuels speculation that the rumored Surface mobile device, currently dubbed Andromeda, might be closer to a reality. And just to be clear, as pointed out by a Reddit user, the PRODUCT_MOBILE_CORE was not the definition given to Andromeda, as it has yet to have one assigned in the SDK.
If the assumptions are correct, this is the latest indication that Windows 10 Mobile is being phased out. Given the popularity of mobile devices, it is likely that Andromeda is more than a rumor, perhaps even making an appearance at CES beginning January 9 or the Mobile World Congress in late February.
Either way, RIP Windows 10 Mobile.