A new leak suggests that the upcoming Call of Duty 2021 title will be set 30 years before Black Ops Cold War time.
According to @ModernWarzone, a known leaker of info anything about Call Duty, he shared that the next game will be set in the 1950s. At that time, the known war event going on is the Korean War, so it could mean that this will be set 30 years before COD Black Ops Cold War time.
No other specifics were shared, but if this leak is to be trusted, then this could be a good setting for the game. The Korean War lasted from 1950 to 1953, which would have a lot of events to cover with the upcoming game.
Take this with a grain of salt for now though. No official confirmation yet for what Call of Duty 2021 will be set on.