Let’s imagine you’re a modern bank and associate card transactions with the merchant.

Even if you sanitize/delete the account holder record, you can’t delete the transactions themselves (as it would screw up your accounting) nor the merchant association.

Now let’s say you have a very particular customer that always goes to a very specific restaurant at pretty much the same time every day consistently.

That pattern alone is enough to single out that particular person (and deanonymize them by correlating with other records such as cell phone location data, etc).