But does this actually still get used by parental controls?

Some parental control applications will look for it but it is not yet legislated to be mandatory on a majority of user-agents.

All I am suggesting is we legislate the header to be added to URL's that may contain material not appropriate for small children and mandate the majority of user-agents the ones that are default installed on tablets and operating systems look for said header to trigger optional parental controls. Child accounts created by parents on the device should not be able to install alternate user-agents or bypass the controls (at least not easily). Parents should be guided through this on device setup.

Indeed their site is old and rarely touched. The ideas and concepts have not changed. It really could just be a static text site formatted in ways that law makers are used to or someone could modernize it.