Another angle is to compare with similar games or updates. For instance, other entries in the Call of Duty series have patches that address specific issues. If this update was for a Warzone mode, it might include crossplay adjustments or new map areas.
Another possibility is that the user is referring to a game update that happened on 2021, January 21st, hence 21.1, or January 21st being the first update of the year. But that's speculative. call of duty 4 modern warfare update 21.1
Since I don't have specific information on update 21.1, I need to state that. However, I can provide a general idea of what typical updates for that game include. It's also possible the user made a typo, such as meaning update 1.21 or 2.1. In that case, suggesting that they check the correct version number or provide more context could be helpful. Another angle is to compare with similar games or updates