You remember, Pocket Infinity. Only the most fun fusion rifle in Vanilla Destiny! But, it's dropped so far off the radar that some guardians don't even know that it's in the game. I'm guessing the main reason why Bungie is keeping it gone is because of the slow burst glitch, where your pocket infinity would shoot its bursts extremely slowly instead of in rapid succession. But all they have to do is bring it back and everyone will be happy with it, glitches or not. At least that's my opinion. It would help a ton if you showed this to your friends and see what they think. The more answers the more accurately the answer can be determined.
It may have allready been in the game, in the book it says complete all 4 exotic quests refeers to Gjallahorn Khovostov Thorn, and possibly Pocket? Or another hidden exotic. I just feel like it with because it fits with this dlc, like a Siva design