Before the era of USB-C devices, external accessories had to be MFi certified. It came with an SDK for communicating between the device and the accessory over the lightning connection, and using ethernet-over-usb was explicitly forbidden for accessories.
But with the new USB-C devices (iPhone/iPad), it's really not clear to me what is the new way of doing accessories.
To be honest, it seems like it is not clear to Apple: I have been contacting different support departments, and they keep sending me to the other department ("that's an MFi question, ask to the MFi support" -> "that's not an MFi question, ask the other support" -> loop).
Does somebody know what's happening there? Is Apple completely dropping MFi and allowing accessories do whatever hack they manage to do to communicate with their app? Seems like it.