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I'd overlooked that note in the documentation before, and in
fact developed the opposite view on what was allowed by
seeing that the binary pickle format happens to allow
persistent IDs containing non-printable ASCII characters.

Given that the non-binary format can represent strings
(containing any character, printable or not) by escaping
them, it seems unfortunate that the same escaping was not
applied to the saving of persistent IDs.

It might be helpful if the documentation indicated that the
acceptance by the binary pickle format of strings without
restriction is not to be relied upon, underlining the fact
that printable ASCII is all that's allowed by the format.

Personally I'd like to see the restriction on persistent IDs
lifted in a future version of the pickle module, but I don't
have a compelling reason for it (other than it seeming to be
unnecessary). On the other hand, it seems to be a limitation
which hasn't caused much grief (if any) over the years...
perhaps such a change (albeit a minor one) in the
specifications should be left until another protocol is
introduced.