When the game makes its transition to free to play. It will become available on the Epic Games Store and no longer available on Steam.
Nintendo has accidentally revealed the release date for Rocket League going to free to play.
Per the original blog -
Anyone who has played Rocket League online before the launch of free to play will be rewarded with Legacy status, which includes:
All Rocket League-branded DLC released before free to play
- "Est. 20XX" title that displays the first year you played Rocket League
- 200+ Common items upgraded to "Legacy" quality
- Golden Cosmos Boost
- Dieci-Oro Wheels
- Huntress Player Banner