To be fair to those projects, the type was introduced only three years ago:

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".

Yeah I don't doubt it was the best option. Just a bit surprised.