343 Industries will be sunsetting Halo Legacy games, December 2021. Players who owns the games will still be able to play them, however online services such as online matchmaking will be limited or disabled.
“Every cycle spent maintaining and supporting Xbox 360 legacy services is time and energy not spent on the future of Halo. To that end – today we are announcing our intention to sunset our Halo Xbox 360 legacy services to fully focus on the future of the franchise. In approximately ONE YEAR, December 2021, online services for legacy Halo Xbox 360 titles will be discontinued. Owners of these titles will still be able to play these games indefinitely, however, certain online features and experiences, particularly online matchmaking, will be limited or disabled.”
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Source: Halo Waypoint