Earlier this week Gears fans figured out how to trigger Warzone—it has to be done from a private Team Deathmatch, while hitting the A and B buttons simultaneously in the match's parameters menu. Warzone's exclusion and mysterious reappearance led to rumors the mode was going to be included later as paid DLC, but an Epic Games multiplayer programmer told Eurogamer that was not the case.
Peter Knepley told Eurogamer that "Execution" is the Gears of War: Judgment mode meant to deliver the gameplay style of the old Warzone. Knepley said it "was easier to leave Warzone refs than rip out," which explains why gamers can see "Warzone" as a game choice if they follow the match creation instructions seen in this YouTube video.
We've contacted an Epic Games representative for comment and will update this post if any is made.
[Update] An Epic representative confirms that "we have no current plans to release Warzone, but we don't plan to remove the stray code either. With that said, it's not an optimal play experience. We did release a very similar mode, Execution, as a free add-on this week. We recommend people stick to that."
Gears of War: Judgment Warzone mode found on the disc, and it's playable [Eurogamer]