[dragonwriter]

> You can even encode data as a side channel on the power lines themselves.

USB-C PD standard basically does this (well, on a side channel compared to the main data lines, at any rate.)

> But that would be more expensive and since the separate data lines were already there few designers thought to prioritise security over simplicity and cost.

Pretty sure that the reason the pre-USB-C quick charging non-standard implementations that used existing data lines didn’t do so because it was cheaper to build but because it was more useful for users to not have to have special, incompatible cables for charging.