I would be surprised if they do not make "opt out", not "opt in".
But the whole reason “pay for ad-free experience” model isn't dominant is that advertisers, in fact, are willing to pay more for exactly the people that would pay the most to opt out of ads (because those are the people with the most money), not pay more for the people who would opt in to ads. So the premise is known to be the opposite of the truth.