> This strikes me as a bugfix that does not get backported because code might depend on
> the bug. If the policy for exception messages, such as it is, documented somewhere?

I don’t know if it’s written anywhere, but the rule I follow is that even though exact error messages are not specified by the Python language, they should not be changed in bugfix releases (I seem to remember bugs for argparse and tarfile where this was invoked).  The commit policy seems in flux these days however, with code cleanups going in stable versions (urllib and collections recently).