Microsoft has sent out an email to Windows Phone developers, informing them that after December 31st, Windows Phone 7.x devices will no longer be able to unlock for app testing purposes. This move is intended to convince app developers who are still coding Windows Phone 7.x apps (for some odd reason) to update their apps to the latest operating system (Windows Phone 8.1).
For those of you who are not familiar, developers are able to unlock their device for app testing. This allows app developers to hit a button in Visual Studio while coding the app, allowing them to test their app right on their phone. This is a fantastic tool for app developers, allowing them to see first-hand what their app will look like and how it will perform on the device. You can read the full email from Microsoft below.
Further reading: Microsoft, Windows Phone
Windows Phone Developer,
Starting December 31, 2014, it will no longer be possible to unlock Windows Phone 7.x devices for app testing. We recommend that you unlock your Windows Phone 7.x devices prior to December 31, 2014 to continue testing with those devices for another 24 month.
After December 31, 2014, no Windows Phone 7.x unlock capabilities will be available. This means, for those of you who have not unlocked your Windows Phone 7.x device, testing of apps directly from Visual Studio on that device will no longer be possible. However, app testing can still be done on other unlocked phones, or via theWindows Phone Emulator, or by submitting beta apps through the Windows Dev Center.
Please note that this change will not impact apps available in the Windows Phone Store, nor will it impact customers with Windows Phone 7.x devices. Additionally, if you have apps that only have a Windows Phone 7.x package, now is a good time to update these apps to Windows Phone 8 or 8.1.
Windows Store Team