[cyphar]

I'm not sure why you'd want PFS for backups. The idea of backups is that you have a history (not just a simple mirror) and so having PFS intentionally renders older backups unusable (unless you're storing all the keys -- in which case you have somewhat defeated the point of PFS).

Now, PFS would allow you to handle key compromise by making future backups unreadable. But there are other solutions for this (such as upgradeable encryption).