I see your point. But nobody ever mentions "thong lines"- I see plenty of those through skirts and snug pants, and why are they any better than "panty lines"?
Well, they aren't.

If you can see someone's thong through their skirt... they need to put a slip on.

And if the pants are so thin and tight the thong outline can been seen... those pants are probably too small... or so poor quality they never should have been purchased.

My undergarments are not seen by anyone... I always make sure of that. Young girls though, don't always care or know any better.