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FO31 defines 'use-any' while XSLT30 defines 'unspecified'. I think this is related to #29929
Thanks for pointing this out (and sorry for not responding sooner!) Erratum E2 has been drafted. It advises implementors to support both "unspecified" and "use-any", with "unspecified" being deprecated.
The WG reviewed accepted this subject to a change: instead of saying that "unpecified" is deprecated, we should say that "use-any" is preferred.
HTML version of the erratum can be found on GitHub, here's the direct link to E2: https://htmlpreview.github.io/?https://github.com/w3c/qtspecs/blob/master/errata/xslt-30/html/xslt-30-errata.html#E2 Note that the erratum was accepted.