If you are following a guide, tutorial, or documentation example et cetera, please provide context. It looks like your reference is stale.
From the 5-9/package directory, I typed this to find any cases of it in the system:
bb@openacs-a22:/var/www/5-9/packages# grep -R user_objects
acs-subsite/tcl/package-procs-oracle.xql: from user_objects
acs-subsite/tcl/package-procs-postgresql.xql:-- from user_objects
acs-subsite/www/admin/group-types/delete-2-oracle.xql: from user_objects o
acs-api-browser/www/plsql-subprograms-all-oracle.xql: from user_objects
acs-lang/sql/oracle/upgrade/upgrade-4.7d5-4.7d6.sql: from user_objects
bb@openacs-a22:/var/www/5-9/packages#
user_objects is only used in comments.
The code in acs-subsite/tcl/package-procs-postgresql.xql suggests the query has been replaced with a simple case of select 1 from dual;
.
Maybe look at these documents for alternatives:
https://openacs.org/doc/object-system-design#object-system-design-api and
https://openacs.org/doc/tutorial-database