It’s not a great verification, but if the idea was that no files should have been deleted, look at the logs:
Moves will look like delete and add, but Examined file count and size seems like they should be stable.
The COMPARE command can compare the view before-and-after rearrange. Might take some looking.
You could have made a separate backup earlier with clone-and-edit config to keep the “before” version.
That’s still sort of an option, but you’d need to restore (good to practice this anyway), then do a backup.
Or if you think it’ll be lots of years, maybe even safer would be just a more future-proof copy or .zip file.
Verification of no-change-but-reorgs could probably be done with a tool specialized in finding duplicates after you do restore of the before-and-after views. In theory all you need is a sorted list of all file hashes, which should be exactly identical. The problem with any of these ideas is making sense of any issues…
which means check your retention policy to make sure the valued version doesn’t get chopped out soon.
Another approach to preserving it is to archive the Duplicati backup files, and preferably its Database too.