It was not unexpected that Microsoft will stop pushing even Release Preview Ring Builds to the Windows 10 Mobile Redstone unsupported devices (Check supported devices list). Microsoft has now made it official after pushing Windows 10 Mobile Build 14393.5 to Release Preview Ring. In a Feedback Hub post Microsoft makes it clear that,

  • Unsupported devices will not receive any builds or updates newer than 10586.x
  • Unsupported devices will not be able to select any Windows Insider Preview rings in the Windows Insider app
  • Updates to apps and services may not be available for unsupported devices and in some cases may limit certain experiences
  • Unsupported devices can still use the Windows Device Recovery Tool to recover to Windows Phone 8.1, but will not have the option to return to Windows 10 Mobile.

Read the full post below,

Hello Windows Insiders!
Earlier this year when we launched Windows 10 Mobile, we announced a series of changes to Windows 10 Mobile Insider Preview Program, including details of our launch of Windows 10 Mobile as well as changes to the supported Windows 10 Mobile Insider Preview devices. 
With the upcoming release of the Windows 10 Anniversary Update, the Release Preview ring is moving to Windows 10 Mobile Insider Preview build 14393. This change will have some impact on unsupported devices:
As stated before, unsupported devices will not receive any builds or updates newer than 10586.x
Unsupported devices will not be able to select any Windows Insider Preview rings in the Windows Insider app
Updates to apps and services may not be available for unsupported devices and in some cases may limit certain experiences
Unsupported devices can still use the Windows Device Recovery Tool to recover to Windows Phone 8.1, but will not have the option to return to Windows 10 Mobile.
As always, we appreciate our Insiders and thank you for your participation in the Windows Insider Program.
Sincerely,
Your Windows Insider Team
@WindowsInsider