when compared to the bootable JAR EAP server provisioned by wildfly-maven-plugin in bootable JAR mode.

The file is standalone/tmp/auth.

Amending the description:
jdenise@redhat.com noticed the issue is that the directory is no longer present in bootable JAR generated by (eap|wildfly)-maven-plugin. In XP 5/EAP 8.0 (eap|wildfly)-maven-plugin didn't handle bootable JAR mode meaning this cannot be regression.