I love Rocket League, i think the game is amazing and one of the great games i played that year. However there were talks about cross play between all platforms and MS did announce that they will support Cross play with any platform and even the Devs themselves said..

Psyonix vice president Jeremy Dunham says work on Rocket League's proposed cross-network play between PS4 and Xbox One is now complete, could be up-and-running within "a few hours", and only requires Sony's approval to be released.

http://au.ign.com/articles/2016/07/20/rocket-league-ps4-xbox-one-cross-network-play-now-ready-for-release-only-needs-sonys-approval

I was thinking about jumping back on Rocket League for some games with my friends. Does anyone know if this has been implemented yet?

No, but you have Crossplay with PC/PS4 and PC/XOne !! There is no reason from a business standpoint for Sony to enable Multiplayer with Xbox ! It would only benefit them and i believe if Xbox wouldnt be that far behind, they would have never allowed it imo.
Also even if it happens, you wouldnt be able to add Friends from PSN/XBL, it would just be random matchmaking with PS/Xbox Players, except for private lobbys where you can join via Password.
So yeah, it wouldnt even be that big of a deal, but still Sony would help Xbox more and "damage" their own and there are more than enough players in like every Game on PS4 that there is no need to.

I’m looking at it as a consumer perspective not a business perspective. XB vs PS is what gamers wanted. I doubt it would affect Sony or MS in any way apart from letting gamers finally play together. The work has already been done for both XB and PS thanks to Psyonics.

As for if Xbox was in the lead they wouldn’t do it? That’s an assumption and not really a point but just a guess. MS stated they support Cross Play on all platforms so we are going with that because that’s whats been stated.

I am guessing from your post this hasn’t been implemented yet.

Don't think it ever became a thing between XB and PS. Kinda sucks you can't do cross platform matchmaking anyway.

I'm not sure and I also don'the really care since I can already crush both X1 and PS4 players.
What they should do is fuck both Sony and MS and put up their own session tracking servers and allow anyone to play with anyone even in a party. There is really no need to connect the networks directly to each other.

That would be a really good idea

Yes it is. Sadly Psyonix hasn't had a good idea since they released the game last year.