Because Apple hasn’t given any word on when it will return. Not sure why you were expecting it to return in the official version… but Apple specifically said they were delaying external display support and it won’t be included in iPadOS 16.1.
M not so surprised that it was pulled out from the latest release. The external display support had a lot of bugs, including OS crash (resulting in all apps being closed) and scaling issues in apps when dragged across screens.