I'm not sure that excluding a volume shadow copy is actually what you need to do. I thought I was in the same boat as you... Windows Backup and Restore was reporting failed backups due to malware. The only references I could find in Windows Defender was to a path similar to Device\HarddiskVolumeShadowCopy5\Download\something.crx, and searching similar paths on my actual drives wasn't turning anything up.
You can use PowerShell to add a wildcard pattern for all HD shadow copies to Windows Defender's exclusion list. The Defender settings GUI doesn't let you use wildcards, but the PowerShell command does:
Defender doesnt have an option to allow for vss "threats". In my case, they are not threats, they are nirsoft files I have already excluded in defender, but when they are being backed up they show as threats in vss and there are no exclusions allowed for vss, only quarantine and remove, so you need to exclude those files from backups altogether
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