Support for XML Extender is still provided in V9. IBM has told me that "support" may be discontinued in V10.
XML Extender is an add-on application that is separate from the database engine, so it would probably still work even if not supported (but you would have have access to the binaries from an older version).
XML Extender basically consists on a bunch of C programs, Stored Procedures, and Triggers (that get generated by the C programs).
I certainly would not recommend using it if you have not started (I have inherited a database that has it).