From Halopedia, the Halo wiki
Season 5 is an upcoming season for Halo: The Master Chief Collection, expected to be released some time in January 2021. 343 Industries intends to release the previously-cut AKIS and Mariner helmets as part of this update, after they were pushed back from Season 4 due to difficulties encountered during development.
Season 5 is expected to add the following content to the game. Notably, the Halo: Reach and Halo 3 customization options were implemented into the game in the Season 4 December update, as available for preview in the game's menus. However, they are unable to be unlocked and used in-game without the use of modding.
For Halo 3, the existing "New Skins in Halo 3" toggle will be updated to include the new armor permutations mentioned below alongside weapon and vehicle skins. If the new cosmetics are turned off, any players using Halo Online-based armor will instead display as using the default Mark VI.