Simple resistor signaling wouldn’t work for current sharing; two EVSEs with a single 40A supply [32A continuous] can advertise 32A if only one is in use and switch to 16A if both are in use. (Same concept extends to a larger pool of EVSEs which can limit to a max total current across the pool.)

Tesla's Mobile connector supports different pigtails for the input circuit. That way it can support anything from a regular outlet (NEMA 5-15) up to something more common for an electric range (14-50).

The mobile connector detects which one based upon resistance.

As others have mentioned, this is often too simple for EVSE to Vehicle connections, as that might require the power level to be dynamic. Load sharing, limitations in supply capacity, or heat may all be potential reasons to limit available power.

It's based on a square wave duty cycle, so you'd need something more like a '555. But yes, J1772 is designed to be dead simple and cheap to implement.