It’s not the same thing. I’d love something like <meta> tags where the websites declare their cookies, with a standardized set of metadata. We could have a browser native permission system like we have for microphone, camera, geolocation, and the possibility to allow or disallow cookies in a unified way. Blocking the undeclared/disallowed cookies would then be done at the browser level, so there would be no need to trust that the websites actually respect the settings

I'd like something like an <ad> tag to go along with it, where the contents are sandboxed by the browser separate from the rest of the page. Mainly as a cudgel against sites which are very anti-adblocker.

Yeah it was by Microsoft https://en.wikipedia.org/wiki/P3P

It was only a failure because it was not mandatory to obey it. GDPR missed a big chance to do that.