You must have an Xbox Live Gold subscription to play Modern Warfare 2 on Xbox One and Xbox Series X/S. As on PlayStation consoles, you can only check out the campaign and local co-op modes without a subscription. However, online multiplayer and Spec-Ops online will both be unavailable to you.
The only COD that you can play without a Core subscription is Warzone: //xbox/games/call-of-duty-warzone Besides that, every other COD and paid games, like FC24, GTA V, etc.