If you delete both your spending key AND its viewing key off of a device then nobody can learn information about the transactions exposed by that viewing key, even if they watched the blockchain all along, sniffed your network connection all along, and they subsequently steal the device on which you previously had your spending key and/or viewing key. ("Forward secrecy")
from Zcash Wiki
So if they don't have the keys they can't break the secret? Even if they have a device, where the keys ar not there? No wonder everybody is confused about crypto!
Forward Secrecy is a different beast: it means you can't compromise a past communication if you get into the possession of the main keys. Simply because there was a nondeterministic temporary session key generated and thrown away after the session.
But what exactly did you try to say here?