> Practically, I am not sure about the feasibility.

Last week I logged into my e-gov website by confirming a push notification on my authenticator app that's tied to my e-ID card to download my birth certificate. No copy pasting needed. Random websites now even support logging in without passwords by using your smartphone passkeys.

I am sure they can make it work for a simple yes/no (over 18) challenge-request scenario.

> Plus, not everybody has a smartphone. Not everyone accepts Android or iOS at their current implementations. Do those people remain without access to the www?

We are talking about adult content. The alternative is having that content banned. Many democratic countries have already done that, like South Korea. I don't prefer this scenario.