I know Nintendo Online isn't required to get the game, but is it required to play the online modes? The servers are run by Psyonix so I would assume no, but you never know with Nintendo. I don't have a Switch myself yet so I can't test it out.
According to a blog post on Rocket League's website (posted in 2020 when the free-to-play update was announced), a paid subscription for Nintendo Switch Online services is not required to play online for Rocket League
PlayStation Plus on PlayStation 4 and Nintendo Switch Online will no longer be required for online play on those platforms.