RTALabel https://www.rtalabel.org/page.php is over 3 decades old. Restricted to Adults, pretty easy to remember.

For some history and related standards, see Wikipedia:

PICS https://en.wikipedia.org/wiki/Platform_for_Internet_Content_...

POWDER https://en.wikipedia.org/wiki/Protocol_for_Web_Description_R...

ASACP/RTA https://en.wikipedia.org/wiki/Association_of_Sites_Advocatin...

The more "digi-ID so we are sure you are old enough, bitte" keeps been pushed, the clearer it's about tracking and not about children. No matter how much they love to frame it the other way around. Unless they want to admit they are total inepts.