From Halopedia, the Halo wiki
The Rhine armor set was first introduced into Halo 3's MCC port along with six other new armor sets, as an Exchange-exclusive armor set in Season 6: Raven. It was never available in Halo 3 for Xbox 360.
The RHINE armor set was originally created as a useable armor permutation for Halo Online, a cancelled Halo 3-derived multiplayer game developed by Saber Interactive and Innova Systems which was intended to be released exclusively in Russia for the PC. After the game's cancellation in 2016, the armor was one of six sets added to Halo 3 in The Master Chief Collection as part of the game's sixth season in 2021 as an Exchange exclusive armor set for 20 season points.
List of appearances