Why does the USB-C plug shell carry a voltage?
It's from an interference suppression capacitor between the shell and the mains AC. The capacitor provides a path for high frequency noise to be shunted to the power supply, but it also allows a tiny amount of current to pass through it. Because your multimeter is very high impedance, it registers as a high voltage, but if you connect it to something the voltage will drop a lot. Your observation of a slight electric shock fits with this as the capacitor won't allow enough current to provide more than a slight tingle.