After reading PEP3101 and the archived mails, I think it's by design. Even if not, it might be too late to change.

I don't prefer either way, emitting warnings by compiler or linter. They can't give hints for runtime retrieved patterns, and not everyone turns on warning and uses linters.

I prefer using a customized string.Formatter, implementing my own `check_unused_args`, raising appropriate exceptions, and used it across my project instead of the default `str.format`. But there seems some problems with string.Formatter.