Infinity Ward developer has confirmed on Twitter that the very first teaser and hint at Warzone was shown back in May 2019, when the first trailer for Call of Duty: Modern Warfare released.
The portion of the trailer which shows the scene starts at the :50 second mark in the video.
Here’s a few images from the moment, which was actually not in the campaign (if you played through it).
And here’s a clip inside of the helicopter:
Call of Duty: Warzone was only officially announced and confirmed on March 9, 2020, when the company announced that it was releasing free to play on all platforms.
Call of Duty: Modern Warfare released on October 25, 2019 with Campaign, MP, and Special Ops modes. Infinity Ward never even hinted that Battle Royale was coming during the year, even at one point suggesting to media outlets that they are only focusing on MP through the year.
Here’s the trailer: