A folder in a shared drive can support up to 20 levels of nested folders.
We do not recommend creating a large number of folders in one shared drive. Users might have difficulty organizing and finding content. Instead, you can organize content into multiple shared drives.
You could work around like this. Assuming you are syncing to a top level folder called backup. When you hit this limit, you could create a shortcut back to another top level folder called deepfolder1. Repeat for any other deep folders.
I think this will work...
PS 20 folders is really deep. Even I don't have any of those and I have the messiest file system you could possibly imagine!
I guess I'll stick with restic (con: zfs snapshots on local and remote get complicated, it's then actually nested 'fs'-es, not just folders representing them).