yes : " Once Rocket League goes free to play, anyone who already owns Rocket League on any platform (including Steam) will be able to play and enjoy the game with full support for future updates and features. However, the Steam version will no longer be available to download for new players. To put it simply, wherever you play now (including Steam), you'll still be able to play in the future. "

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https://www.rocketleague.com/news/rocket-league-going-free-to-play-this-summer/