UPDATE: Epic has offered an update on the confusingly worded statement issued to press earlier today regarding Rocket League's future status on Steam, now insisting that it has "not announced plans to stop selling the game there".
In its initial press release, Epic wrote that "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." That combination of In the meantime and thereafter reading as if Rocket League's sale on Steam would continue only until it transitioned to the Epic Games Store later this year.
In a new statement issued to US Gamer, however, Epic has said, "We are continuing to sell Rocket League on Steam, and have not announced plans to stop selling the game there...Rocket League remains available for new purchasers on Steam, and long-term plans will be announced in the future." All of which feels a lot like a game of semantic evasion, but for now the takeaway is that Rocket League may or may not continue to be sold on Steam post its Epic Games Store debut. Make of that what you will.