Call of Duty: Warzone is down for maintenance until 10 am PT (6 pm GMT) on Monday, November 28 to make way for the launch of Warzone 2. It will return from maintenance on November 28 as Call of Duty: Warzone Caldera.
The original CoD: Warzone isn't going away permanently. Which is confusing, given that Activision has been officially referring to the new game as Warzone 2.0. But no, it isn't replacing the old one. The Warzone you've been playing up until now temporarily went offline on November 16 and, around ten days later, will come back under a new name: Warzone Caldera. All existing guns and cosmetics will persist in Caldera, though the Rebirth Island and Fortune's Keep maps will not be returning.