It does now, and there are lots of other reporting options and several third party tools to build on that base. Although a backup that fails due to an abort due to a missing folder doesn’t make a log, it can send email:
Failed: The source folder C:\missingfolder\ does not exist, aborting backup
Details: System.IO.IOException: The source folder C:\missingfolder\ does not exist, aborting backup
at Duplicati.Library.Main.Controller.ExpandInputSources(String inputsources, IFilter filter)
at Duplicati.Library.Main.Controller.<>c__DisplayClass14_0.<Backup>b__0(BackupResults result)
at Duplicati.Library.Main.Controller.RunAction[T](T result, String& paths, IFilter& filter, Action`1 method)
How would one distinguish between a missing folder and a missing share or a missing file, or is this a general feature request to be more persistent in getting what it can, maybe issuing a warning instead?
–allow-missing-files is kind of a crude solution. Sometimes missing things are normal, sometimes not. Duplicati doesn’t know, but you might, and if you’re into scripting (sounds like you might be), you could –run-script-before to create a filter to exclude things that went missing that you’re willing to just forego.
https://github.com/duplicati/duplicati/blob/master/Duplicati/Library/Modules/Builtin/run-script-example.bat which is also on your system installation shows how you can do this. I think there are forum posts that accomodated USB drives this way, and you might do similarly. Or just use --allow-missing-source and forgive whatever goes missing (which might be a step too far). Also note that missing sources can’t be backed up, so make sure your retention rules don’t allow all of the versions of the share to age away…