Update: It should be pointed out that this was a temporary thing and has been restored.
Cross-platform play in the Nintendo Switch version of Rocket League has been disabled. The feature won’t be available for a few more days until the software update version 1.44 is rolled out. The date and time for the upcoming patch’s arrival will be announced soon by developer Psyonix, so stay tuned for any updates. In the meantime, you could still play the game online with other Nintendo Switch players.