Season 5: Anvil
From Halopedia, the Halo wiki
Season 5: Anvil is a season for Halo: The Master Chief Collection, released on January 27, 2021. 343 Industries included 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. Additionally, Season 5 sees the introduction of 12 armor sets from the cancelled game Halo Online into Halo 3.
The season is named after ANVIL Station and Anvil Initiative; a space station known for testing the MJOLNIR armors introduced with the update and a the organization that runs the station, respectively. The season's logo is the Anvil Initiative's logo: a Human and a Sangheili shaking hands.
Season 5 added 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.
Halo: The Master Chief Collection
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.
While Season 5 launched on January 27, 2021, the patch released in December of 2020 physically added the bugfixes and new content to the game. The patch notes for the December patch can be found here.
January 27 patch notes (Version 1.2094.0.0)
The Season 5 patch launched alongside the start of the season on January 27, 2021.
Install Size by Version
Season 5, “Anvil,” begins with this update. It includes a variety of new unlockable cosmetics for Halo 3 and Halo: Reach, including new skins for armor and weapons, new nameplates, and a new Mongoose skin. The New Skins in Halo 3 toggle in settings has been expanded. Players who want to maintain a classic Halo 3 experience can flip this setting to “disabled” to avoid seeing any new cosmetic items while playing Halo 3, including weapon skins, vehicle skins, and armor. While cosmetics are disabled, any players in your game who are wearing new armor will appear with the default Mark VI.