2.6 million player-created game types from Halo 3, Halo 4, and Halo: Reach were successfully transferred for future access in Halo: The Master Chief Collection.
Revealed on Halo’s Twitter, this transfer was the first one that will be followed by custom maps on August 26, 2019, at 10am PT/1pm ET.
For those unfamiliar, this is a one-time service that will bring over custom game types and maps from the original Xbox 360 versions of Halo 3, Halo 4, and Halo: Reach.
Source: IGN.com 2 Million Custom Game Types Copied to Halo: MCC from Halo 3, 4, and Reach