It seems that Game Pass subscribers can look forward to more Call of Duty coming to the service soon. Both Insider Gaming and XboxEra claim to have sources indicating that 2009's Call of Duty: Modern Warfare 2 will be added to the service in March or April.
It's worth remembering though that this may not be the original game, but the remaster of the Modern Warfare 2 campaign that launched in 2020 for PlayStation 4 and Xbox One. If it is the latter, then it is only the campaign and no multiplayer that applies.
Assuming the information is correct, it will be the first older game in the series to come to Game Pass. As it stands, 2023's Call of Duty: Modern Warfare III and Call of Duty: Black Ops 6 are available to all war-hungry subscribers.