LongSensation a very well know and reliable leaker has revealed that Battle Royale will be coming to Modern Warfare.
It is said to be coming in 2020 post launch of Modern Warfare, and it will be a free to play and download for all players.
So apparently Battle Royale in #ModernWarfare will be a free mode to release in early 2020. It will be downloadable as a separate game, but also will still be connected to the core game. If this is true then it's a very good move by Activision.— LongSensation (@LongSensationYT) August 7, 2019
LongSensatio as also stated that the Modern Warfare BR will be it's own entity while it's still connected to the core game itself.
The game will be released October 25th of this year, and most likely we will see the release of the Battle Royale sometime early 2020.