Proton doesn't really support anonymous payments. Let's demand that they do

Nostromo:

Are we sure Proton is storing this info

Yes, the value is visible on the pdf invoice that is stored associated with the account on Proton’s account page. Who else would be storing it?

Account → Subscription → Invoices → View/Download

Nostromo:

it can indeed be correlated if someone wanted to?

I know two things:

1. Proton retains the gift card card as history in your account once redeemed. I’m hesitating posting an image containing my own previously redeemed GC codes publicly.
2. Proton generates and sends the gift card code via email on purchase. Though it has been some time and I can’t recall exactly how this value is transmitted, but at some point the user is provided this value to be redeemed.

It does not seem like it would be a stretch that these two events could be correlated based on some unique key. (The gift card code).

Obviously #2 is required, but if the code was ephemeral and was not retained with the account, then this would break the chain.