When I clicked on the link to codebases relying on the specific error string, I was expecting to see random side projects. Wasn't expecting to see Grafana and Caddy on the list.
replies(3):
https://github.com/golang/go/pull/49359/files
Before that, doing a string compare was basically the only way to detect that specific error. That was definitely an omission on the part of the original authors of the stdlib code; I don't it should be classified as "Hyrum's Law".