Nah it's not an exploit. It's using your exotic slot tactically. Equipping a sidearm and switching is more of an exploit, but that's more Bungie's fault at overlooking it. But taking advantage of Icebreaker's exotic perk is not exploiting anything. I would say that using a Y1 weapon should be less effective in power-enabled playlists like Trials though. It shouldn't be a 1-shot kill at 170 attack.