With the addition of PlayStation 4, Rocket League is now cross-platform across all major platforms including PlayStation 4, Nintendo Switch, PC and Xbox. Rocket League is now the second game to take part in the PlayStation cross-platform beta program, with Fortnite being the first.
The feature is enabled by default, but can be toggled on and off in the menus. This is a great first step, but you might be asking about cross-platform party, and that feature will launch with the first big update coming a bit later.
This is heading the right way, so what do you expect is the next game to get cross-platform support?