RMAN-08591: invalid archived log deletion policy

Not documented that I could find, however, it appears that when using an RMAN configuration to ensure archivelogs are applied to the standby before deleting them, RMAN relies on the data guard broker to take care of this check.
If you were to turn off the log transport via dgmgrl and run your RMAN backup, you would see the following error at the end of the output when deleting archivelogs:
RMAN-08591: WARNING: invalid archived log deletion policy
Looks as though RMAN depends on DMON when using this policy in the configuration:
CONFIGURE ARCHIVELOG DELETION POLICY TO APPLIED ON ALL STANDBY;