This was discussed at length previously here: Be part of our cashback pilot 🎉
Basically, there was a fairly sizeable backlash over the privacy aspect of cashback, and Monzo took steps to make it more clear as to what data is shared and how.
I believe now the app has one or two informative screens before you agree to enable cashback that have pretty clear text explaining how it all works - did you not see those when you enabled cashback?
Of course, at the end of the day, enabling it means your transaction data will be shared and it’s everyone’s personal choice whether to proceed or not.
I myself was against it at first, but after Monzo’s remedial action and my thinking that I don’t really use Monzo for spending (so there is very little transaction history to share), I opted in favour of it. But everyone will be different.
With regards to de-identifying customers, anyone who has ever delved into it knows de-identification techniques are mostly useless.
The spending history shared is all of it (not just the merchant for whom you are enabling cashback), this data goes to the aggregator to be then used to give you more relevant cashback offers; it’s early days, and we have yet to see what that means (in the latest round of cashback offers, I don’t think anyone got anything they’d consider ‘personalised’).
The data sharing occurs when you enable cashback, at least that’s my understanding of the process based on the explanations and documentation I’ve seen.