I like the idea, but I haven’t see any product do it, so would be surprised if Duplicati does it now, or even is engineering in a way that can wait for backup data to become available.
Azure Archive tier is only accessible by demoting specific blobs from hot or cold tiers, and once the data is there it is not readable. The blob has to be requested to be moved up to a hot or cold tier, and that is measured in hours.(Reference: Azure hot, cool, and archive storage for blobs | Microsoft Docs, Archive Access Tier section)
My impression is (and please correct me if I’m wrong) that Duplicati is using cloud storage similar to backup to disk. Since it is an incremental forever backup product, being able to access all previous backup files/blobs is important. A typical archival/full+incremental backup product would be better suited to take advantage of archive tier, but not an incremental forever.
I also suspect that the amount of development required to move backup files up and down between tiers would be significant. For instance, to restore a file would require selecting a file, submitting a request to Azure move the correct backup blobs from the Archive tier to Cold tier, then queuing up the restore and periodically checking to see if the backup blobs are available in the Cold tier.
I haven’t looked at the structure of Duplicati internally enough to know whether this is a big leap, but given that I have yet to run into a paid backup product that includes this functionality, I’d be surprised.
Just my two cents.