According to the announcement, “With special thanks to all of our friends and colleagues at PlayStation, we are thrilled to announce that starting RIGHT NOW, Rocket League has entered the PlayStation Cross-Play Beta program! Players on PlayStation 4, Nintendo Switch, Xbox One, and Steam can now randomly matchmake with or against each other in all Online match types (i.e. Casual, Competitive, and Extra Modes).”
Harmonix notes that cross-play will be enabled by default, but in case you had previously turned it off, you will find the option to re-enable it via the Options menu under the Gameplay tab. Cross-play support for the PlayStation is currently in beta so expect that there will be some kinks, but hopefully nothing too jarring.
Sony had always been somewhat resistant to the idea of cross-play, but they made an exception last year with Fortnite presumably due to the game’s immense popularity. There are other games such as Minecraft which Sony had also previously refused to enable cross-play on, but hopefully this decision with Rocket League means Sony is more willing to play ball these days.