I think the idea of a "jule thief" basically is a extremely low power boost converter to run a LED as a nightlight on the remaining few % of an otherwise deplet

I think the idea of a "jule thief" basically is a extremely low power boost converter to run a LED as a nightlight on the remaining few % of an otherwise depleted battery. A standard switching power converter wouldn't be called that way, even when modern chips used for that purpose obviously try hard to be as power efficient as possible. In a typical circuit nowadays you'd also avoid the use of a two coil transformer, and just have a "one coil" inductor to save on copper. The second winding is needed to control the.transistor in the simple circuit shown on Wikipedia. Also the simple circuit doesn't provide for any regulation. The more elaborate gameboy schematicsuses mentioned in the parallel comment to mine uses feedback (look for the 4.7v Zener pulling down the transistor) to accomplish that. But of course nowadays one would use a simple 4 to 8 pin IC integrating almost everything except the inductor.