Please do not use Migros Cumulus, Coop Supercard and similar discount systems. Please do not use free credit cards either. It is an absolute disaster for your personal data (privacy). You have no privacy at all, the data is traded and sold.
Please only pay with cash and without a loyalty card. Anything else is grossly negligent.
Could you link to evidence of widespread, negative effects for consumers caused by use of loyalty cards and credit cards?
Privacy and security itâs a complex topic.
Please do your own research
https://www.kuketz-blog.de/ (DE)
Ad-Free Privacy Tool/Service Recommendations - Privacy Guides (EN)
I donât think it makes a big difference if you pay by card. Your credit card is linked to the receipt. Whether your credit card is linked or/and Cumuls doesnât make that much difference*. But of course, somewhere there is a database containing all the items you have bought (also visible in the Migros app). Either because it is stored for Cumulus, or because the receipts are stored for 10 years. If you donât want this, you need to pay in cash and without Cumulus.
And yes, such things are requested by public prosecutors if you are under investigation (source: I was once at a lecture where a lawyer, I think it was Martin Steiger, said this).
*Except that you may have granted Migros more rights when you signed up for the Cumuls card. Which could of course be a problem, but such data is usually used for market research and not sold on as raw data - at least I have the feeling that this would otherwise be known in the media.
Well, we allow them to trade our data for a % refund from our sales I guess⊠I am aware of these issues and still use these cards for the benefit they provide, I donât think it is negligent as long as you are aware of it.
Privacy is a problem everywhere⊠from the web browser and the websites we visit to the places were we walk and travel. In my opinion this is something that needs to be be regulated by gouvernments to avoid wrong practices⊠From the consomer perspectives, avoiding all possible threats to privacy is very complicated without too many constraints in your lifestyle.
This is a bit off topic anywayâŠ
You apparently already did research that led you to your claims of gross negligence, so it would be great if you could contribute it to this discussion. Instead of asking people to find proof for your claims 
why can Coop and Migros monitor for a tiny discount? In the ALDI and LIDL, the prices are massively lower, and you do not have to be monitored. This discussion is very strange.
Not everyone has an Aldi or Lidl nearby.
Just saying, you still get 1% of Lidl if you let them monitor you. Whether you want that or not is another question.
But I agree that I donât see the point in bringing down Coop prices by 1% if there is a Lidl next to it (which often isnt).