Info on the Official Site
Since the news broke Steam reviews for Rocket League have dropped it recent rating from "Very Positive" to "Mixed"
Currenty this the only information regarding the future of Rocket League on Steam:
Editor’s Note: We wanted to clarify something for you after today’s news: Rocket League is and remains available on Steam. Anyone who owns Rocket League through Steam can still play it and can look forward to continued support. Thanks!