Already happens in New Zealand: lighting is usually low current 1mm2 wiring, and everything else is heavier gauge. Circuit breakers mostly care about Amps (all breakers could be rated to mains voltage if you wanted to avoid “weird”).

Also low voltage wiring can legally be done by anyone in NZ (a bonus when doing your own work, and a pitfall when buying a house?)

Maybe for you, but I have been considering just this. I would love to have dedicated 24v for lighting and charging of devices. My house already has various systems for lighting, such as xenox throughout kitchen under the cabinets and also the basement. Both are driven from separate transformers. Then I got the rest of the house with can lights utilizing br30 bulbs that are just a waste of 12 awg. The one place I was able to replace with dedicated LED fixture, I had to overpay for a decent product that wouldve been better off as a 24v basic LED light. When you consider most hvac systems operate at 24v, there is some real potential to create a decent standard serving multiple purposes.

And besides, idk if you have ever pulled 12ga wire, but it's a pita. Idk any electrician that would agree with you saying it would be a pain to cut back on heavy wire and pull half that with light 22 awg.

A lot of countries already have lighting on a separate circuit. It means that when something trips a breaker you don't lose all your lights as well.

Why? Modern code requires a separate lighting power runs