Can you play Warzone 2 without Modern Warfare 2. 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.
An Xbox Game Pass subscription is not necessary to play. Call of Duty: Warzone 2.0 falls under the free-to-play standards.