Psyonix just detailed the December Update for Rocket League that adds Xbox One X support.
The final planned update for 2018 is the December Update that arrives on December 3. This update adds Xbox One X support which brings Native 4K 60fps (supersampling on 1080p displays) to the Xbox One X. It will also support HDR. Other platforms will get the following with the December update:
Leaderboards for Ranked Extra Modes
Changes to the XP/ Progression System, including more Weekly Win Bonuses
A brand new ‘Rocket League x Monstercat Vol. 5’ EP
New Community Flags for Extra Life, Speedy, and another still TBA
Rocket League is available on PS4, Xbox One, Nintendo Switch, and PC.