Before the update, players are allowed to pick and stalk any characters they want in Quick Play mode as well as Competitive Play mode but Blizzard changed things up with the new update.
While you will still be able to stack your heroes in the Quick Play mode, stacking will not be allowed in Competitive Play. According to Jeff Kaplan, the Game Director of Overwatch, they did that to ensure that the players will have a better gaming experience in Competitive play.
He added that he have seen a few stacking strategies that totally messed up the game before their opponent can do anything about it. They finally decided that it would be better if hero stacking was not allowed in Competitive Play.
If you enjoy those stacking strategies, you can still play them in Quick Play mode. What do you think of the change?