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.
Guess which option will be set to off by users, which youtube will make the default to "on", and then entirely remove the button when users adapt to turning it off.
Surprisingly, it is actually being deployed with opt-out being the default.