I worked for the company that made the "oxidized" cables in the previous photo here in my country. The first (I think) "special speaker cables" in the USA, the Monster Cable, were taken as an example. We upgraded them with red and black caps and gold-plated studs on their ends, back in the days of push-on/bare-wire connectors, used on both amps and speakers.
I still have the ends I cut off so I can use "U" terminals, so I'll take a look at them and let you know.
I bet the inner conductors aren't oxidized because, as you say, if the wires are too tight, oxygen/moisture won't get in.
But I wonder why so many cable manufacturers keep using transparent PVC if its chemical composition helps oxidation, probably because the appearance is attractive, seeing the inside of the cable is like seeing the "soul" of the sound for many audiophiles. You see that I don't care, I also listen to music closing my eyes when I want to concentrate....