All The PlayStation Plus Tierse\n\n Subscribers gain access to online multiplayer, monthly games, exclusive discounts, and a few other benefits. Extra  Essential tier benefits + a catalog of downloadable PS5 and PS4 games. Premium  Essential & Extra tiers benefits + a catalog of classic PS3, PS2, PS1, and PSP games.

Yes, you need a PS Plus subscription in order to play Modern Warfare 2. Without this service, you will not be able to access the online multiplayer and online Spec-Ops missions on the PlayStation 4 and PlayStation 5. If you don't have PS Plus then you can still access the campaign and local co-op modes.