Without any stats, I suspect retouching is so prevalent, that basically Norway mandated every ad put a sticker on the copy, and consumers will just learn to ignore it. Hopefully that doesn't end up the case, because this is a great first step.

It also doesn't seem like there's any way for the consumer to find out the extent of the retouching. Like, let's say I'm interested in a product but the picture is retouched, can I find the original image anywhere to see a more realistic depiction of that product?

One of the complaints I have with Prop65 labels ('this product is known to the state of California to cause cancer' labels) is that its significant extra work for me, as the casual consumer, to figure out what and how much is in the product. So by default I would want to avoid it in general, but if there's only 3 options and they all have the warning, I can't tell if one is straight cancer another just has a little cancer dusted on and the third uses a much less cancerous alternative chemical that still falls afoul of the marking laws, but is barely harmful if used as directed.