Yes, I agree with the @PeteM description and the quoted help text. The 2017 discussion is a bit dated.
Test smart retention policy #3008
We had some good discussions how the feature works, we have introduced that backups outside the specified time frame get deleted, and we had some more weeks with no complaints that anything was seriously broken. This should be sufficient to let this optional feature pass to “experimental”.
Feature/issue 3008 retention optimizations #3026
As discussed in #3008:
--retention-policy now deletes backups that are outside of any time frame. No need to specify
--keep-time as well. Help text was changed accordingly.
--retention-policy is now mutual exclusive to the other retention options
--keep-version. Help text was changed accordingly.
--retention-policy now complies with
--allow-full-removal, deleting even the most recent backup, if it’s outside of any time frame
It’s automated if you can express it with one of the retention policies, e.g. are willing to say how long the backup should retain old versions of files to guard against unwanted deletions or changes. If it’s noticed soon that some files got backed up that you don’t want, an easier and safer-than-purge way is to delete entire unwanted backup version right then with The DELETE command. The latest version is always 0.
This method isn’t selective, so make sure that you leave enough recent backups to feel comfortable for other files, but from a storage point of view it might be more attractive than typing up space a long time.
Purge is fast too, but if you mis-aim a purge-these-files-in-all-versions you can seriously harm a backup.