"Devices not on this list will not officially receive the Windows 10 Creators Update nor will they receive any future builds from our Development Branch that we release as part of the Windows Insider Program. However, Windows Insiders who have devices not on this list can still keep these devices on the Windows 10 Creators Update at their own risk knowing that it's unsupported," said Windows Insider chief Dona Sarkar in today's blog post.
Microsoft attributed the short list of support phones to Insider feedback that indicated older phones might not be providing "the best possible experience" for customers (I've heard a number of testers running phones not on this list claim that the Creators Update test builds have worked well on their devices, and in some cases better than Windows 10 Mobile Anniversary Update does, for what it's worth.)
Microsoft also released a Fast Ring test build of Windows 10 Mobile for phones to Fast Ring Insiders today. That build number is 15204 and it includes a number of bug fixes. This is the first Redstone 3 build for Windows Phones. It's only available to Insider phone users of handsets that are on the list above.
Sarkar said that Microsoft will continue to roll out Windows 10 Mobile test builds as the company moves ahead with Windows 10. However, the build number and branch won't match the builds Microsoft will be releasing for PCs, Sarkar acknowledged.
Sarkar attributed this strategy to "more work we're doing to converge code into OneCore - the heart of Windows across PC, tablet, phone, IoT, HoloLens, Xbox and more as we continue to develop new improvements for Windows 10 Mobile and our enterprise customers."
In addition to releasing the new Windows 10 Mobile preview build today, Microsoft also released Windows 10 Build 16176 (Redstone 3) for PC testers in the Fast Ring. Like the first Redstone 3 build Microsoft rolled out last week, today's PC test build does not include any major new features or changes, as Microsoft is continuing to do work under the hood on OneCore with code refactoring in preparation for check ins of new code.