> Xerces2 Java has supported this stuff since around 2010.
>

Indeed so (Saxon's SCM has also been around a long while). I decided in Saxon
to deliver the SCM as an XML structure rather than a Java structure because I
reckoned many potential users would be happier accessing it using XQuery/XSLT
code rather than Java code; and also because I wanted a transport/interchange
format for compiled schemas.

Saxon also offers a direct XPath interface to the SCM as a function library:
see
https://www.saxonica.com/documentation12/index.html#!functions/saxon/schema

Michael Kay
Saxonica