No, I'm wrong, looks like it is part of a standard.
USB Power Delivery (USB PD) 1.0 supports 9V as one of its four voltage types, along with 5V, 15V, and 20V, at power levels up to 100W. Some USB cables can also support 9V, such as a 1 meter USB-C to USB-C cable that can support 9V/3A.
It's...