Google has just removed three devices that were offered as vanilla Google Play Editions: the Sony Xperia Z Ultra, the LG G Pad 8.3, and the 2013 HTC One. These devices have been removed from the Google Play Edition page and also from the official support page.
While there are a number of possibilities as to what happened to these devices, the most probable one is Android Silver. Android Silver is a set of regulations, requirements and guidelines that Google will set for future top-end Android phones. If manufacturers adhere to these their devices will be branded as ‘Android Silver’ devices.
While Android Silver won’t be replacing the Nexus line, the devices will be running vanilla Android, so perhaps the other 3 remaining Google Play Edition Devices, the HTC One M8, the Moto G, and the Galaxy S4, might also be departing soon.