Effective in version 10.0, you can use
pmrep purgeVersion -c
with or without the -p option.
When you use the -c option with the -p option, the output lists the object versions that purge, then lists which object versions are contained in deployment groups.
When you use the -c option without the -p option, the command does not purge versions that are part of deployment groups.
Previously, when you used the -c option, the -p option was required.