EDIT: digital cameras didnt work, until it did. streaming didnt work, until it did. ecommerce didnt work, until it did.
You can't play whack a mole with the internet. People will always find a way to move smut or whatever on the internet. It takes no time at all to spin up more and more sites, and there's a million ways around them (vpns, etc).
All it does it just push people to more and more fringe sites, when moderation is likely to be lax and the content more extreme. Ideally it wouldn't be viewed at all, but it's just how the internet is.
It also sets a terrible precedent for censorship- in the UK, we've already seen, on Reddit for example, subreddits dedicated to quitting addictions being age gated, and it'll only get worse.