IIRC it's not in the spec because administration of resources is out of scope. For example, perhaps you offer a public repository and you want folks to sign up for an account before they can push? Or you want to have an approval process before new repositories are created?

Regardless it's a huge pain that ECR doesn't support this. Everybody I know of who has used ECR has run into this.

There's a long standing issue open which I've been subscribed to for years now: https://github.com/aws/containers-roadmap/issues/853