Apple has been historically fickle about how it lets marketers and developers track iOS users through apps downloaded from the App Store. After all of the privacy concerns were raised about the UDID device identifier back in 2011, a better solution never presented itself.
Apple eventually introduced its own Advertising Identifier for iOS device tracking purposes, but marketers still favored the unique, permanent nature of the UDID. The UDID worked so well because it was a device-specific identifier that could never be changed. Athough developers were technically banned from using the UDID to track iOS devices more than a year ago, many, many apps still use the deprecated method today.
Apple is reportedly starting to reject apps that use web cookies to track user activity in iOS. Could this mean a reinvigorated push towards the Advertising Identifier again?