What I found especially amazing by that is traditional accounting is based on immutability already: you never delete a ledger entry, you add a correction. The same concept for a blockchain is trivial. Equally, if you _really_ do need to delete something, just insert a record into the blockchain instructing all nodes to delete that record. Sure, you won't be able to verify the chain 100% after you delete the data, but you'll have a auditable record of why that's the case.