Rocket League is now available for cross-platform play on Xbox One, Nintendo Switch, and PS4. Psyonix made the announcement yesterday in a post on the Rocket League blog. Sony allowed Rocket League to play with other platforms via the Playstation Cross-Play Beta program. Here are the instructions as noted in the blog post:
"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). To make sure that Cross-Platform play is enabled (it is by default), head to the "Main Menu," select "Options" and then make sure the "Cross-Platform Play" box is checked in the "Gameplay" tab. And that's all there is to it. You are now ready to play against gamers on other platforms across the world!"
Coming soon, Psyonix will release an update to Rocket League, introducing their cross-play platform party system to allow players to team up with their friends on any supported platform. For now, you can still play with your friends by creating or joining a private match in Rocket League. Psyonix hopes to give Rocket League players full cross-platform support by adding more platforms in the future.