If such a feature were specified then a site would merely have the option of providing alternatives. In the event that it did, whether or not to follow such redirects would be entirely up to the client.

Such a system is clearly the technically superior solution. It regulates the provider as opposed to the client, forcing the market to provide a workable solution for concerned parties while the client maintains complete control over how things are handled. It further steers well clear of any slippery slopes by not mandating the broadcast or collection of personal information.

Perhaps important from a liability perspective, it places the onus on the client as opposed to these latest attempts to shift it squarely onto the service provider. Right now the legality of serving content across jurisdictional boundaries is extremely convoluted. With ID or age reporting laws it clearly becomes the service provider's responsibility. In contrast, a mandatory metadata standard for classification would create a situation in which it is clear that the legal responsibility (if any) to appropriately configure filters falls to the client.

Of course such a solution would be of no help to the anti-porn and pro-surveillance lobbies. That's the entire point.