JBoss EAP 7.3 Configuration Guide
A.31. IO Subsystem Attributes
https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.3/html/configuration_guide/reference_material#io_attributes

NOTE
... See the schema definition file located at EAP_HOME/docs/schema/wildfly-io_2_0.xsd to view the elements as they appear in the XML, as there may be differences from the management model.

We should update the schema version from wildfly-io_2_0.xsd to wildfly-io_3_0.xsd.
JBoss EAP 7.3 standalone.xml refers to the version 3.0 schema as below:

<subsystem xmlns="urn:jboss:domain:io:3.0">
     <worker name="default"/>
     <buffer-pool name="default"/>
</subsystem>