Amazon has made it so that users of the Amazon Fire TV with the pendant design can no longer use the official Amazon Fire TV Game Controller. The controller is incompatible with the Fire TV device after the latest 22.214.171.124 software update for the third-generation device is applied. Scuttlebutt suggests that the Fire TV Cube will also soon lose support for the gamepad.
This move wasn't an accident with a new disclaimer at the bottom of the page for the Fire TV Game Controller that reads "not compatible with Fire TV with 4K Ultra HD - Pendant Design, Fire TV Edition smart TVs, or Fire TV Cube." That leaves the only device that is compatible with the Fire TV Game Controller as the Fire TV Stick. The Fire TV Stick is still on sale for $25 for Prime subscribers right now.
It's unclear why exactly Amazon has removed support of the controller from those devices. Reports indicate that if you have a Fire TV 3 that is running software version 126.96.36.199, the previous version of the software, the Fire TV Controller still connects and works without issue. Another odd tidbit with this is that while the disclaimer says that the Fire TV Game Controller doesn't work with the Fire TV Cube, it is still working as of the latest 188.8.131.52 software according to reports. That particular Fire TV Cube update started rolling out to devices this week.
Older devices do still support the game controller including Fire TV 1, Fire TV 2, Fire TV Stick 1. The only current generation device that has official support for the controller is the current generation Fire TV Stick 2. As for why support is being dropped, there is no definitive answer from Amazon. Speculation is that it has to do with the complexity of maintaining support for the odd Wi-Fi Direct connection method the controller uses. It's unclear if Amazon has a new version of the controller in the works that will support current devices.