343i will remove online matchmaking for all legacy Xbox 360 Halo games in 2021, forcing users to buy the Master Chief Collection to continue playing.

After 14 long years of support, 343 Industries is finally removing online matchmaking in legacy Halo games on Xbox 360. In December 2021, 343i will shut down online servers for Halo 2, Halo 3, and Halo 4. At that time all online matchmaking will be unavailable, and file shares for these games will be wiped out too. Service records for Reach and Halo 4 will be wiped, but curiously Halo 3's and ODST's will remain intact.
Offline modes like local split-screen co-op, campaign, firefight, custom games, and forge will continue to be supported, of course.

This applies to backward compatible copies of the Halo games. These games won't be exempt even if you pop in an Xbox 360 version on Xbox One or Xbox Series X/S, After December 2021 gamers will only be able to play these Halo games online via the Master Chief Collection.
As a long-time Halo fan, this moment is bittersweet. But I knew it was always coming. Once 343i took over the Halo franchise they significantly reduced stat tracking anyway--the Halo website's current stats are very sparse compared to the original Bungie.net--and Microsoft has to move on at some point.
It's still great to see Microsoft support four generations of Xbox games even as it pushes ahead into the new generation.