I did a bit of research. Looks like the <meta name="rating"> tag is supported by Google's Safe Search. Most adults probably do Google searches with "safe search" turned on (since we don't want porn sites most of the time) and this will put website owners in a dilemma. What if it's not for kids but you don't want to drop out of Google search results? That's going to discourage usage of this tag by anything other than porn sites.

I asked ChatGPT about browser support for the meta tag. It appears to be an experimental feature in Firefox 146 that's turned off by default [1].

So, there's some work on this feature, but it seems like another signal is needed to say "It's not porn but I don't want my website to be visible on devices that have parental controls on," which would be needed for it to get mainstream usage.

Also, often you won't want to drop out, but just redirect kids to more appropriate content. For example, Lego's website has a popup to redirect kids to the "play zone." It might be nice to do that automatically, but the <meta name="rating"> tag isn't going to do the trick.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Exp...