You remember right, shutdown-image was added by quisvir
in PRS+ 2.1.01a and should still be present in PRS+2.1.02a
At least the code in StandbyImage.js
holds all necessary information.
Though this option will be (temporary) removed with PRS+ 2.2.x, as kartu
wants to build 2.2.x on stable 2.0.18.
This is also reflected in my PRS+2.2 clone
- which I would NOT recommend in any case!