Rocket League is suffering from a little platform fatigue at the moment. The most recent update for the game has gone live on every system except the Nintendo Switch. It takes more time for patches to be certified for the Switch than it does for those on PlayStation 4, Xbox One, or PC. Thus cross-platform play has been suspended temporarily on the Switch.
Can't have people pre-update playing with those who have it after all! Psyonix promises that they will keep fans updated on when cross-play will be returning to the Nintendo Switch. Their hope is that they will be able to offer version 1.44 to Switch players in the next few days. This will of course update the Nintendo version to the same as its competitors and re-open cross-play.
Psyonix has reassured Switch players that they can still play online, but only with other people enjoying Rocket League on the same platform. This is a bit of a bummer, but considering Rocket League is one of those games we have to thank for normalized modern cross-play, let's give them a break. Keep your eye on all the official Rocket League social medias and other platforms over the next few days to see when the Switch version of the game will be updated.
Sources: My Nintendo News & Twitter