Earlier quoted context omitted.
I fail to see how you can get this kind of data from anonymized supermarket receipts? Entirely different data collection methods: sure. But the entire point of this is that was too difficult ("time-consuming and error-prone", according to the SSB).
Issue national ID cards that contain a private key. For each citizen, the government would encrypt a mapping of their national ID number -> anonymized ID ("anonID") using their corresponding public key. The anonID could only be used to look up anonymized demographic data, updated when new IDs (and thus new encryption keys) get issued, once every few years. When checking out at the supermarket, your national ID number…
One example of this is when our national newspaper (NRK) did an investigation[1] on "anonymized" GPS data, and could easily find the identities of the persons they belonged to.
[1]: https://www.nrk.no/norge/xl/avslort-av-mobilen-1.14911685