Implement RS485 or I2C protocol using the usb-c connector?
I don't think anything prevents you from doing that, but it is common sense to not to use a connector for something else than it is already meant for.
At some point cables may get mixed and somebody will connect it to another real USB device by accident and it can damage your device or the other device unless you plan for accidental connectivity. Even if nothing gets damaged, somebody could still expect it being a USB device since it has the USB connector.
The impedance of USB cables is nominally 90 ohms, which is somewhat smaller than nominal recommended minimum of 100 ohms and quite a bit smaller that the typical 120 ohms.