Rocket League players on the Nintendo Switch may have noticed that cross-platform play with other consoles has been disabled. This is only temporary and online play is still functional. It will be a few days before cross-platform play comes back as it should be returning when the next software update rolls in. The time and date of when update 1.44 will be announced by Psyonix soon.
Here is the announcement by Psyonix by Twitter regarding Switch cross-play.
Hi Kieran! Yes, Rocket League players on Switch can still play online, but only with other #RLonSwitch players. Cross-platform play being disabled on Switch is only temporary until v1.44 is live on Switch in a few days. (Date and time to be announced soon.) https://t.co/4OaTCgluOh
— Rocket League (@RocketLeague) April 11, 2018