Psyonix have been working on Rocket League cross-play for a while now and while it was supposed to go live in September 2018, it had been delayed. There was no ETA following the delay, but info suggests it will come at some point in 2019.
Cross-play has been the talk of the town for months now, with Epic Games spearheading the get together movement with their hit title Fortnite. Other companies' willingness to support cross-play ranged on a scale from Sony to 10, but they all eventually come aboard.
Psyonix never really intended to deprive Rocket League fans of cross-play, but RocketID is unfortunately not coming within the next couple of months. Technically, the game already has cross-play, but they can't party up before a match with their friends who are on a different platform. That's where RocketID comes in.
This will be a standalone system that will allow players from different platforms to party up. While it should have been released already, Psyonix's most recent roadmap update informed fans that RocketID will not be ready this year. It is now set for launch in "early 2019".
The reason behind this might be Sony's recent decision to give in to cross-play demands, as Psyonix didn't intend to include PlayStation 4 in cross-play support. The developers stated that once RocketID launches, it will indeed support all the platforms Rocket League is available on.
To wrap the RocketID announcement up, Psyonix stated that they will aim to deliver a stable, polished and intuitive service at launch. Psyonix will definitely have their work cut out for them, as they also announced that Chinese launch for Rocket League is "fast approaching", but there is no officially confirmed date yet.
On top of that, the studio announced Frosty Fest 2018, which will be a seasonal event starting in mid December 2018. According to the post, the event will be structured fairly similarly as the currently ongoing Haunted Hallows event. You can read the full statement on the official website.