Has the bottom port of the Wii U Gamepad been used for something?

The Wii U has a port between the two charge terminals on the bottom of the controller. Has this port been ever used for something? (like official or 3rd party accessories or modifications)

A Reddit post claims:

It's an accessory connector/port that was never used. With the Wii U doing so badly in sales, Nintendo and third parties never made any accessories for the Wii U gamepad. This picture shows how it could have been used.

The post includes a link to a picture that shows something like a joystick accessory connecting to this port (which never went to market):

The official Nintendo User Guide calls this port an Accessory Connector, and multiple sources claim the same. Because of the unpopularity of the product, no accessories were ever made to utilize this slot (none that I could find at least).