The heists are firmly locked away in there terabytes of files only to be used once people stop buying in game cash with real money.
They may also just wait till the next gen version comes out, but I doubt that........I highly doubt that.
R* is a good company and has put out some epic games. But when it all comes down to it they are just another company looking to make money. GTA V sold hella good and still is, even more so now because people have already fully pre order NG versions.
Anyway back to the heists. As I said, it's all about the money. They have made it RETARDED to earn money the right way in that game, the economy is utter shit.
E.X. You play a lvl 5 mission:
Yay, you beat it, here is 1500. Go buy your self something nice.
Hm, you need ammo or a new gun? Well that new gun you want is 25000 so keep grinding that mission.
So a lot of people have just started giving R* 10, 20, 60, even 100$ more just for in game cash that is gone just as fast as the bought it. So, what do people do? They buy more cause they want the best guns, cloths, cars, houses, etc.
So in the end, the heist DLC is NOT the biggest priority for them right now, and why the hell should it be? There getting real money up the ass cause people keep paying for in game cash and keep buying the game.