Nintendo isn't the most tolerant of game companies when it comes to letting fans make derivative materials or post lots of videos of Nintendo games online. One prime example is when Nintendo killed off the browser-based Mario 64 remake back in 2015. More recently it forced the indie developer making Super Mario 64 an online multiplayer game to remove all his videos from YouTube.
Nintendo has a Creators Program in place that allows people to make videos including Nintendo IP in them, but Nintendo gets paid all the advertising revenue for monetized videos. It then shares that video with the gamers using a 70% cut for channels and a 60% cut for individual videos. Nintendo has made a big change to that Creators Program that will leave some participants unhappy- it no longer allows live streaming.
Nintendo writes, "Live streaming on YouTube falls outside the scope of the Nintendo Creators Program. You cannot broadcast content on YouTube Live from the account you have registered to the Nintendo Creators Program. If you plan to broadcast content on YouTube Live, you have a couple of options. First, you can broadcast content on YouTube Live from a channel that is not registered to the Nintendo Creators Program. Or, you can cancel your channel's registration to the Nintendo Creators Program and instead, register your videos containing Nintendo’s IP to the program separately. Videos which had previously been registered through your channel would need to be re-registered individually."
If you read the passage above, you will undoubtedly notice that it doesn't say "no live streaming of Nintendo games," it says "you cannot broadcast content on YouTube Live from the account." That would seemingly mean you can't live stream anything from your account, even if the game isn't a Nintendo IP. If your channel is registered, and you want to livestream other games, you will need to unregister your channel and register your videos individually. The catch is that you will lose out on 10% of that revenue for having your videos registered only.
There is no indication of why Nintendo doesn't want its channel partners to live stream video. Perhaps with big streamers like PewDiePie getting in hot water for the things they say and do in their videos, Nintendo just doesn't want its family friendly IP tied to a platform where bad things could be said in realtime. Granted, a video could have not so friendly language in it as well. There has been no official statement from Nintendo on why this change has been made as of writing.