From Halopedia, the Halo wiki
Steam rewards are attainable items awarded for players who play Halo games on Steam. They are commonly divided into Trading Cards, Emoticons and desktop wallpapers. Currently, the only Halo games to support Steam Rewards are Halo Wars: Definitive Edition, Halo: Spartan Assault and Halo: The Master Chief Collection. Emoticons can now be unlocked using Steam Points in the Points Shop.
Halo: Spartan Assault
Halo Wars: Definitive Edition
Halo: The Master Chief Collection