Not exactly. CrashPlan had a special retention option for deleted items, but Duplicati doesn’t that feature. Instead Duplicati keeps the entire backup version for a certain amount of time (depending on your retention policy). It never considers individual files within a backup job separately when it comes to retention.
The Smart Retention policy might be fine as it keeps versions for up to 1 year. But note it doesn’t keep ALL backup versions for that 1 year. For the first week, only one backup is retained per day. Beyond that, for the first 4 weeks it only keeps one backup per week. And beyond that, for the most recent 12 months it only keeps 1 backup per month. So if a file only existed on your system for 1 week and then was deleted, it may not be recoverable after the first month depending on exactly when it was deleted. Instead of Smart Retention you can craft a custom retention option that errs on the side of safety more, if that’s your desire.
Nope. As mentioned above Duplicati is talking about entire backup versions, not individual files. There isn’t a way to tell Duplicati to keep 1 version of each file forever. Telling Duplicati to keep only 1 backup version means that when you complete a backup job it will delete the older backup versions. So if a file gets deleted, and you do another backup, that file will become unrecoverable.
Duplicati isn’t really an archive system, it’s a backup system. I personally don’t recommend you delete any source data as then your only copy is in Duplicati and it’s no longer a “backup.”
You can use its integrated email notification feature, or you can use a 3rd party service like duplicati-monitoring.com.