Update: A clarification was sent out earlier today where Epic said that they won't stop supporting Rocket League on Steam, as they never could actually do, since legions of players that had already purchased the game on that platform would pick up their pitchforks with a vengeance. However, wording on Epic's clarification leaves much to be desired, and seemingly confirms that the game will not be available on Steam:

"The PC version of Rocket League will come to the Epic Games store in late 2019. In the meantime, it will continue to be available for purchase on Steam; thereafter it will continue to be supported on Steam for all existing purchasers. (...) "Rocket League remains available for new purchasers on Steam, and long-term plans will be announced in the future."

Epic Games has announced the acquisition of Rocket League developer Psyonix, which created one of the most addictive non/Battle Royale game of recent times. The move by Epic will see the games- introduction to the Epic Games Store, with platform exclusivity confirmed for late 2019 / which means that anyone looking to purchase the game on a PC/centric digital storefront will have to go to Epic's, since Steam will be leaving the vendor equation.