Trying this with Duplicati version 2.0.6.3_beta_2021-06-17 results in an error “Setting multiple retention options (–keep time, --retention-policy) is not permitted”.
Just for reference, I found another discussion, that --retention-policy was changed 2018 as follows:
--retention-policynow deletes backups that are outside of any time frame. No need to specify--keep-timeas well. Help text was changed accordingly.--retention-policyis now mutual exclusive to the other retention options--keep-timeand--keep-version. Help text was changed accordingly.--retention-policynow complies with--allow-full-removal, deleting even the most recent backup, if it’s outside of any time frame
Refer to https://forum.duplicati.com/t/removing-files-from-backup/13014/3
P.S. It is a good practise to let old threads stay unclosed, allowing to add such referrals years later.