Now, it has been confirmed officially that Rocket League will be getting a cross-platform update soon, thanks to a tweet from the official Rocket League support Twitter handle. Thanks to Twitter user Floshke, who asked the Rocket League support for any news regarding the cross-platform play, and Rocket League support replied
The development team behind the game confirmed that they are working on the cross-platform support for the game and 2018 is the release window for it as of now.
At present, there will not be a full-mix of the cross-platform play. Voice chat will not be present across platforms and secondly, those who want to play cross-platform can do so via invite only. The quick match will not be cross-platform, i.e. only private matches will be cross-platform.
Hey there! We’re actively working on cross-platform party support for a 2018 release.
— Rocket League Help (@RL_Support) January 21, 2018
For the cross-platform play, there is no word from Sony, whether they will be joining or not.