Sure. But... like... git doesn't know anything about deprecated repos. AFAIK, that's not a feature of git's repo fetch machinery. Anything Github would do to address this would have to modify the contents of the repo, right?
> ...go get for example would need some sort of structured metadata [to do reasonable repo deprecation warnings]
I mean, the JavaScript development community has -collectively- decided on a huge bundle of ad-hoc standards. I bet that it would be trivial for the signatories of the open letter to decide on a tagging mechanism to use in their README files to indicate repo deprecation. Do you disagree?