I suspect that the reason it hasn't been enabled yet is because Apple isn't yet requiring developers use the new API. They don't want people turning on a switch, then getting nagged by an app that hasn't been updated and wondering why the switch didn't do what they expected it to.
It's only once Apple forces all developers to use the new API and starts enforcing it with app review that it'll be worth having the switch there, otherwise you'll change the setting but still get nagged by apps that show their own messages rather than using the new review API.