I was doing some testing on 8.1.0 OnPremise and the good thing is, Microsoft is handlign this quite nice (i am not sure if this was the case from the beginning of the audit log).
I created a field "Bank Secret" in the account entity with auditing enabled, created a new account and afterwards deleted the field from form and entity again.
Result
- The audit-entry still exists
- the field name is flagged with a id and a "[deleted]" marker
- The value is removed and shows the famous "audit" icon
Cheers