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If you are a Windows Phone app developer and want to continue testing your apps on Windows Phone 8.0, you might want to take heed of an email Microsoft is sending to app developers today. After 7 p.m. Pacific time today, Windows Phone 8.0 devices won't be able to be unlocked for development purposes anymore.

Here's the text of what Microsoft is sending to app developers:

The Windows Phone 8.0 certificate that validates developer unlocks will expire today May 13, 7pm, Pacific Daylight time. We recommend that you re-unlock your Windows Phone 8.0 test devices prior to today 7pm, to continue testing for another 24 months.

After today, no Windows Phone 8.0 device will be able to be developer unlocked, and you will have three options to continue testing Windows Phone 8.0 packages: – Update the phone to Windows Phone 8.1 and unlock it for developer testing – Submit apps as beta through the Windows Dev Center and test with Windows Phone 8.0 devices – Test Windows Phone 8.0 apps via the Windows Phone emulator

Any Windows Phone 8.0 device currently unlocked will continue to be unlocked and work as a test device until its certificate expires. To maximize the time available to test with this device, please lock then unlock the device before May 13, 2015 7pm PDT.

Note that this change has no impact on apps available in the Windows Phone Store, has no impact on Windows Phone 8.1 devices, nor will it impact customers with Windows Phone 8.x devices.

This shows that Microsoft is making developers distance themselves from making apps that support the older Windows Phone 8.0 in favor of the current Windows Phone 8.1 OS. Microsoft has already announced that all Windows 10 devices will have access to a "developer unlock" mode in its settings.

Thanks to everyone who tipped us!