If you're a kid and your parent don't give a damn and set you free on the internet you will be exposed to porn.
Why talk to your kids and educate them + maybe do your best to limit their exposure to things you don't agree with when you can clutch your pearls and demand ridiculous things like this?
This will definetly backfire. It's a sad state of afair that reminds me about some dumbass local politicians somewhere that tried to make pi equal 4 since using 3.14 was too hard.
Filter in the browser, at the ISP supplied router, using DNS ... None of it is perfect but also it doesn't require the privacy problems that this does.
Also I have a feeling (although it is just that) that kids aren't getting their porn via google searches. My guess would be social media/chat apps all the time.
Firstly people will just go to the sites which have under 1/3 and therefore are not in-scope.
Secondly sites will game the threshold, if they think it'll be to their financial advantage.
Thirdly underage kids will (I would imagine) work out how to bypass these filters pretty quickly, using VPNs et al.
Fourthly, people attempting to bypass these filters might fall prey to malware dressed up as VPN/bypass software.