Thank you for your review. I commited the patch as r80824 (I fixed the documentation, :versionadded => :versionchanged), blocked as r80825 (3.2).

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> Unfortunately, POSIX says nothing about how to store bad filenames in
> a pax archive. tarfile raises an error. GNU tar fails silently,
> it just puts the unchanged original filename into the pax header
> without converting it to utf-8, thus violating the standard.

Right. I opened a new issue about that: #8333. I consider that it's a different problem.