Nothing works absolutely. You just want to get the 80% that it would be effective for and delay it long enough for the curious to lose interest.
It'll affect PornHub and a small handful of similar sites that are located in the U.S.
Any porn site that based outside the U.S. will not be blocked. In fact, none of these sites are blocked. They only ask the viewer to go through a third-party age verification system to set up an account. The foreign sites will still be available to view without age verification.
You're not getting 80%. You're getting maybe 5% (but these 5% include the most popular ones).