https://www.gamespot.com/articles/new-rocket-league-update-resets-leveling-system-he/1100-6461470/

The new Rocket League Progression Update is live on all platforms. The update changes how the game's leveling system works, adds new achievements/trophies, implements in-game clubs, and patches in numerous online multiplayer improvements. The update also unlocks the game's first Rocket Pass, which lets players unlock exclusive rewards for a price.

As the name of the update implies, Rocket League's progression system has changed the most. Now, XP is only earned in online casual and competitive matches, and the amount you receive is primarily dependent on match length instead of overall score. A match needs to go on for at least 60 seconds to earn any XP, and experience scales upwards with every second that passes before maxing at 20 minutes. You'll still earn small bursts of XP for scoring, being the MVP, playing consecutive games in the same casual server, or replacing a bot in a casual match.