In letter : Xbox 360 Halo games have been out for a while, entertaining us for many hours since their launch. However, starting January 13th, 343 Industries will be deactivating and reducing online services for many Halo games, but that will not prevent you from playing single player modes as well as split screens, Forge and LAN.
The Halo Support FAQ has been update to reflect the services that will be disabled on Xbox 360 Halo games. These include Halo: Reach, Halo 4, Halo 3: ODST, Halo 3, Spartan Assault, Halo: Combat Evolved Anniversary, and Halo Wars.
After January 13, services such as matchmaking, file sharing, MOTD (match of the day), challenges and extras will no longer be available. For Halo 4 and Halo Reach, service registration and player customization will also stop working, but those services will still be available on Halo 3 and ODST.
Achievements in Halo 3, Halo 4, and Halo: Reach will also be impacted, but not all. Only those that require matchmaking, are challenge-based, or are tied to specific activities will stop working.
You should be aware that backward compatible versions of these games for Xbox One and Xbox Series X | S will also stop working. Halo: The Master Chief Collection and Halo Wars Definitive Edition will not suffer the same fate. The titles featured in the collection and the DE use different online services, which means you can still play them as you have so far.
While some may not be happy to read this news, we have to consider that some of these games have been available for 15 years. Since then we have Halo 5 Guardians, Halo Wars 2, upgraded versions of most of the Xbox 360 Halo and Halo Infinite games, so it is not totally unjustified to discontinue support for some online features.