USB-C / USB 3.0 alternate mode as low speed serial port
USB Type-C connector specification does define a &quot;Debug Accessory Mode&quot;, DAM. If both CC lines are pulled down with 5.1k resistors, a USB Type-C device must enter &quot;debug&quot; mode. See Apendix B to USB Type-C specifications.
The standard defines all power and orientation protocols on entry into DAM, but leaves the transport level up to designers, so you can use the other available wires as you wish, UART, LVDS, whatever, proprietary.