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I'm not sure about a central opt-out register. It's an all eggs in one basket affair. At present one of the options we have in our defence is to set up variations on details such as email addresses, physical addresses, dates of birth and all the rest of the stuff to make it more difficult to combine data. A central register means that the user would have to give details of their account on that register to each site with which they connect. It becomes a single point of failure tracking.