If I'm not mistaken, anything after the S4, inclusive, comes with flat cord (but that's just below the headset controls; the cord that goes into each bud is still round). However, in my experience, the ones from the S4, the Note 3, and the S5 have something funky going on with the headset functions, as my old Note 2 headphones worked perfectly to control music on an iPod and iPhone, but the ones from the newer phones don't seem to like the action button and do squat if I press it.
Also, I've read that both the headset and the charger seem to vary from region to region, so it MIGHT be possible that your region always gets round-corded headphones even in newer phones.