It seems as though Call of Duty: Warzone will be adding 200-player modes at some point in the near future. This is all according to an in-game promotional announcement for future content.
You can see a screenshot of the upcoming content announcement for Warzone, which comes courtesy of Twitter user DissolveGaming (as originally reported by VideoGamesChronicle). At the very top left of the promotional material screenshotted by the user, you can see a message in the background that reads "BR 200" and "Plunder 200."
This references 200-player modes coming to both the traditional battle royale mode of Warzone, as well as the loot-based Plunder mode. At the moment, the maximum player count either mode supports is 150 total players. The newly-launched Season 4 added 50 versus 50 games for the first time to Modern Warfare, which is currently the biggest mode in Modern Warfare's multiplayer side.
Although, given past comments by Activision, these upcoming additions shouldn't actually be a huge surprise. Back in March when Warzone first launched, Infinity Ward co-studio head Patrick Kelly stated to USA Today that they were "already playing with 200 players," and that players should expect that "a little bit later."
Right now, there's no indication as to when the 200-player modes could release for either Plunder or Warzone's battle royale modes. We'll keep an eye on this story though, and update it with any further developments or comments from Infinity Ward.