The Go crypto libraries are opinionated, but those opinions are not explicitly presented anywhere. I wrote https://golang.org/design/cryptography-principles as a first step towards documenting them.

I propose we adopt the principles listed in that document as the criteria for choosing what's accepted in crypto/... and golang.org/x/crypto/....

Note that it's a one-page document, so it can't be an extensive policy explaining all decisions, but aims to align and make clear the goals that we are trying to achieve.