Powered by a new version of the IW engine, Modern Warfare II continues to support cross-platform multiplayer and also features a free-to-play battle royale mode, Warzone 2.0, a follow-up to the original Warzone, all within a single cross-game launcher, known as Call of Duty HQ.
As confirmed when Modern Warfare 2 was originally announced, Warzone 2.0 is "the new, free-to-play battle royale Warzone," so although it's built on the same technology that's powering MW2 you won't need to purchase that full-price release to get access to it.