Any thoughts on whether Duplicacy could be used for Archive purposes too rather than backup?
Rather than constantly adding more storage to my NAS over the last few years I’ve been archiving old data to Glacier and removing completely from local storage and backups.
Be nice to include some deduplication and encryption with that. 
Did you mean using Glacier as the storage? Currently there are some difficulties in implementing a Glacier storage, due to the need to pull down chunks for reconstructing latest snapshot during backup, but the Infrequence Access Storage class can be supported with a simple change.
Not so much Glacier but the concept of using Duplicacy for Archiving practice rather than Backup.
Currently I’m testing using Duplicacy as my cloud backup solution for my NAS to B2 Backblaze.
But, when I need to create more space on my NAS, I archive my old data off my NAS to Glacier. But it would be great to use Duplicacy as a solution for my Archive needs too so that I could benefit from the the deduplication and client encryption too (whether that be to Glacier or Backblaze or any other cloud provider).
I think Duplicacy should be good for archiving too. After you delete old data, when you back up new data, the variable-size chunking algorithm used by Duplicacy will be able to identify chunks that exist in the storage (and thus in the old data) and thus be able to deduplicate without the presence of old data.