Crossplay has been a hot topic in 2018, with Epic Games and Fortnite spearheading the initiative, but Psyonix and Rocket League were among the most vocal supporters of joining PS4, Xbox One and PC communities. Sony finally let it happen.
To top it off, crossplay is not only possible between the three mainstream platforms, but Nintendo Switch can now play with all of the cool kids. This sure was not an easy decision for Sony executives, who didn't mind crossplay with PC, but were strongly opposed to letting their players have fun with Xbox One and Nintendo Switch users due to them being direct competition.
Therefore, Rocket League crossplay already existed, but it was kind of odd as PC players essentially had two player bases they could play with - the one where it's only them and PS4 players and the one where there are PC, Xbox One and Nintendo Switch players.
With Sony fully opening the crossplay doors, players are all in one single base now, where anyone can play with any other player, regardless of their platform. Just like in Fortnite's case, players can choose to opt in or out of crossplay on the main menu.
The RocketID system is still in the works though, so crossplay parties are still not available. Psyonix did say in their official announcement that this feature will come with the first update in 2019.
It is interesting that Sony are still calling the PS4 crossplay a "beta program" which means they haven't officially released it yet, nor do they officially support it as a full-fledged feature. Technically, this could mean the company is still working on polishing the system or that they still have a way to abandon it at some point down the road, should an opportunity present itself.
That is all speculation so far though, but the actual confirmed news look way better since PlayStation 4, Xbox One, PC and Nintendo Switch can all play Rocket League together now. It should have even fewer issues than Fortnite, since Rocket League should have fewer crutches for console players that others might find unfair.