This specific sort of filter has been in place in the UK since 2004. It's part of the "Mobile Broadband Group"'s code of practice – the joint industry associati

This specific sort of filter has been in place in the UK since 2004. It's part of the "Mobile Broadband Group"'s code of practice – the joint industry association representing the large UK mobile networks. The principle in general was that mobile broadband operators would enable an "adult content filter" by default on their products, with the ability for users to disable it on providing proof of age. I think it's basically a DNS block, though the implementation might vary between operators. If I remember the details right, I'm pretty sure this was a compromise solution – basically one of those situations where the industry generally agrees to self-regulate in exchange for avoiding more formal state regulation on their services. It's a bit silly generally, particularly when stuff like archive.org gets added to the list when it really shouldn't be.