they're no longer necessary for us and will almost definitely start to
rot now (like commonmark and asciidoc outputs did previously). most
existing users seem to take the docbook output and run it through pandoc
to generate html, those can easily migrate to use commonmark instead.
other users will hopefully pipe up when they notice that things they rely
on are going away.
optionsUsedDocbook has only been around for one release and only exposed
to allow other places to generate warnings, so that does not deserve
such precautions.
f52f531a4e