There are plenty of legal purchases that people would want to keep to themselves. If there were an anonymous non-cash way to purchase legal medications, many people would take that option rather than reveal health details to data brokers.

Maybe my answer is entirely UK based, but I'm pretty sure if I asked in my circle of non tech friends, I don't think any of them would have a concern about buying anything legal on a credit or debit card. You might not want your friends to know, but I don't think they care in the slightest about the bank or CC company knowing. And even if they did, that can be obfuscated a little more by buying it with a non-high street bank (like Revolut) they never have to go into/use for their mortgage or whatever.