According to Rocket League's official website, Psyonix LCC will make Rocket League a free to play game. It has not specified the exact date, however the announcement states that it's happening this summer.
The free-to-play version of Rocket League will be an Epic Games Exclusive, meaning that as the game becomes free, it will be available exclusively on Epic Games Store for PC players. After that you won't be able to purchase Rocket League on Steam, however, those who currently own the game on Steam now will be able to play after the changes take place.
Those who played Rocket League online before the launch of free version 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
- Faded Cosmos Boost
More details could be found in the official announcement.