The Docbook TC is discussing the mid-term plans for Docbook 6, see:
https://lists.oasis-open.org/archives/docbook/201610/msg00005.html .
Just like at Docbook 5, the normative standard will be written in
RelaxNG+Schematron. In the past they generated dtd and xsd files out of
RelaxNG. But for Docbook 6 they consider to drop the xsd version - the
dtd will survive as it can be generated automatically.
* Does anybody need the xsd version in the long term? * xmllint is able
to validate against RelaxNG.
Kind regards, Jürgen