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...
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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.
First they'd need to export the errors the stdlib returns https://news.ycombinator.com/item?id=41507714
I wouldn't hold my breath on that one.