The computer platform that launched with Rocket League on July 7, 2015, was Windows. The game released that day on both Windows and PlayStation 4, establishing its original PC-and-console launch lineup.
Windows was the computer platform named in Rocket League’s initial release information. The PC version helped the game build a broad online audience alongside the PlayStation 4 edition, while Psyonix emphasized fast multiplayer and cross-platform ambitions during development.
Other computer platforms arrived later. macOS and Linux versions were released on September 8, 2016, more than a year after the original launch. SteamOS was associated with planned Linux-based hardware incentives, but it was not the platform listed as launching Rocket League on July 7, 2015.
The date is also easy to mix up with later console releases. Xbox One followed on February 17, 2016, while Nintendo Switch arrived on November 14, 2017. Thus, Linux, macOS, and SteamOS are plausible neighboring answers only if the question asks about later computer support or related hardware—not the original launch platform.