I don't know why the plan to have to opt in for pornography or other adult content was abandoned. It I am sure would have made a difference, and perhaps made some content uneconomic to post.
If you ask websites to classify their own content, there would be an awful lot of 'Biology Education' sites suddenly springing up. If you use bots, they'll let a lot of dodgy stuff through and then block pictures of breastfeeding mums and the like.
If you ask the general public to work together to report sites that show 'inappropriate' material, you'll get a huge amount of keyboard warriors and woke people with a grudge making malicious and/or self-centred accusations (i.e. "I don't like it, so it must be banned"). Look at all of the JKR lynch mob - nobody seems able any more to just thoroughly disagree with somebody or even to try to start an active debate - much easier to just cry 'hate speech' and get them 'cancelled'.
The other problem is that a great many sites host all manner of content. You might have filthyperv.com at one end and fwuffybunnygoestonursery.com at the other, but there's an awful lot of crossover in the middle. MN is full of a shedload of useful, harmless content, but if somebody wanted to pick up on one thread that they personally found objectionable and demand the whole site be classified on the basis of that, where would that leave us?
On the flip side, if a site was classified based on the majority of its output, the porn sites would just start to carry a whole load of old, public-domain, inoffensive material to outweigh the dirty stuff. They might 'discover' that 99.99% of users happen to favour the 'adult' categories over the wildlife, historic buildings, Middle-Ages madrigals categories etc, but as far as they are concerned, they offer a wide range of general interest content, to appeal to the full spectrum of human experience.
I remember reading in the past that some groups tried to campaign against Amazon, because it sold a lot of pornographic/erotic/violent/anti-whatever books. It's everybody's personal choice to use/not use/boycott them for any reason, but I don't think these people could grasp that Amazon just sells every published book that's available. There isn't some sweet little old lady going through lists of millions of titles and deciding what might be nice to sell and what it's probably best to leave out.