Psyonix even mentions Sony by name in the Fall Roadmap, stating that "with Sony's recent announcements regarding full cross-platform functionality, we can now make additional changes to prepare RocketID for ALL platforms should we receive permission to www.lolga.com do so!" It would be nice to know for sure if Rocket League will be given permission from Sony, but it seems like Psyonix might release RocketID without the Playstation players if they aren't given permission.
Another reason for the delay is likely Psyonix's intentions to have as smooth and polished release as possible. Since this is such a highly requested feature, it would be a shame for it to be buggy upon release. Rocket League in its current state isn't without bugs, so players would undoubtedly grow frustrated if RocketID's release isn't smooth.
While it's disappointing that Rocket League Credits cross-platform parties have been delayed once again, it's understandable that Psyonix wants to get it right. Additionally, Playstation players will be happy that Psyonix is so excited to include them in the feature's release. Until then, we can enjoy the Haunted Hallows event, the upcoming Frosty Fest winter event, and the release of Rocket Pass 2.
Rocket League's cross-platform profile system, RocketID, has been pushed back until early next year. Psyonix said that it's working "incredibly hard to ensure that it's as intuitive, stable and polished as possible, before releasing it to the world." When it arrives, RocketID will enable cross-platform parties and friend connections.