Support for RELAX NG used to be enabled together with XML Schema support
(--with-schemas). Now there's a separate option and a new feature macro
LIBXML_RELAXNG_ENABLED.
This option would allow for a smaller, but mostly useless minimal build.
But it complicates the symbol availability logic in an insane way and
requires specialized tools like our custom C parser in doc/apibuild.py.
See #717.
The latest spec for what it essentially an XPath extension seems to be
this working draft from 2002:
https://www.w3.org/TR/xptr-xpointer/
The xpointer() scheme is listed as "being reviewed" in the XPointer
registry since at least 2006. libxml2 seems to be the only modern
software that tries to implement this spec, but the code has many bugs
and quality issues.
If you configure --with-legacy, old symbols are retained for ABI
compatibility.
