Nintendo is extremely protective of their IP, to an extent many might consider “greedy”. If you want to show clips of Nintendo games on YouTube, they’ll take all of your advertising money.
Unless you sign up for their Nintendo Creators Program, where they’ll give you a cut of the ad revenue.
But from now on, you’re not allowed to livestream Nintendo games if you’re part of their Creators Program or Partners program. And it doesn’t matter if it’s monetized or not.
Instead, Nintendo suggests making a new account that’s not part of the Program and livestream from there.
Nintendo didn’t give a reason as to why they are banning livestreams from their Creators Program, but the recent incident with popular YouTuber PewDiePie might have something to do with it (where he uttered a racist word during a livestream).