Windows App Studio

Microsoft will end support for making Windows Phone 8.0 apps in its Windows App Studio tools on Friday, December 17, in an effort to focus developers on creating universal apps that will work with Windows Phone 8.1 and Windows 8.1.

Microsoft says that developers who have made apps with Windows App Studio that target Windows Phone 8.0 can still " the source code, publish package, and installable packages of the apps." However, after December 17, any updates to those apps "will have to be generated as universal Windows project targeting Windows Phone 8.1 and/or Windows 8.1."

Source: Microsoft Thanks to Balaji for the tip!