Hopefully that means delete based on a config. It looks like S3 lifecycle rules are able. Don’t tell it to. 
I’m not sure how it connects to the object lock question, but I agree we need to live with what we have.
Bucket configuration makes me ask – what mode and period? Governance mode seems a little looser.
Setting period near Duplicati retention period might create some benefits, but I don’t know if it’s enough.
Any backup will upload a dlist file and some dblock files with changed data, and dindex files for dblocks.
Duplicati retention directly affects deletion of dlist files corresponding to backup versions, and indirectly affects deletion of dblock and dindex because their blocks are still in use as long as dlist file uses them.
was trying to avoid an extra fee from early deletion. With object lock, you’re avoiding early delete failures.
This is far from a solid path, and you can expect some bumps. Ideally set up an AWS escape hatch too.