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I thought about that myself, too. It is clearly no new feature, it is really more some kind of a fix.
Unicode pathnames given to tarfile.open() are just passed through to the open() function, which is why this always has been working, except for this particular case. There are 6 different possible write modes: "w:", "w:gz", "w:bz2", "w|", "w|gz" and "w|bz2". And the only one not working with a unicode pathname is "w|gz". Although admittedly tarfile.open() is not supposed to be used with a unicode path, people do it anyway, because they don't care, and because it works. The patch does not add a new broad functionality, it merely harmonises the way the six write modes work.
Neither can we retroactively enforce using string pathnames at this point, nor should we let a user run into this strange error. The patch is very small and minimally invasive. The error message you get without the patch is completely incomprehensible.