What part of the standard mandates, or even optionally supports that? Standard USB-PD only requires that you negotiate a voltage and current suitable for both

What part of the standard mandates, or even optionally supports that? Standard USB-PD only requires that you negotiate a voltage and current suitable for both sides (among a preset list). It essentially goes open loop after that; you can't exceed the current limit set, but beyond potentially tripping the PD controller (on the sink side) if the voltage drops too low, there's not really any protection here. It's fully on the PD sink to implement (or, not implement) any sort of protections like that.