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Just saw the notification for this in my Youtube Studio. For now it seems as if this setting is turned off by default, but for anyone like myself who uploads videos to Youtube, I recommend checking that they don't decide to toggle this on for you in the future.
Both could be filtered out in theory (which they should do if they were smart, because amount of training data matters way less than quality or training data), but filtering AI created slop is harder, especially if you just slightly modify it.