Sure... but this is why we have sem versioning and release notes. It's always nice to try and support all users but sometimes you just need to ship breaking changes...

While in principle you're correct, Go the language is very dedicated to backwards and forwards compatibility; while there's been talk of a Go 2 for a long time now, they're not eager to go there and if they do, they intend to make the transition low impact.

That said, I'd say this is an excellent candidate to deprecate or warn about now, and to make impossible in a version 2. Then again, how would you even stop this? A string representation of an error is common in any language, you need it to log things.

I think at best there will be a static analysis rule (in e.g. go vet) that tries to figure out if any matching is done on the string representation of an error.

Hyrum's law is specifically about how every change is a breaking change if you have enough users. So it's always a bit subjective. No sane person considers changing an error message a breaking change in context of semver. It's just go going above and beyond to take care of backward compatibility.