I don't think I've ever seen a button that says "mail me my 2fa code". The workflow basically always goes like this:

1. I enter username/password and click "sign in". 2. Agorithms run on the server. 3. If the algorithms think "suspicious" I'm redirected to an "enter your emailed code" page and automatically send me an email.

In any case, the top of this thread was specifically referring to this type of transactional email.

Taking a quick look at my email history, I have a whole pile of transactional mail (from Canadian entities) with no unsubscribe links: a bank email notifying reception of a complaint, a bank email about my paycheque saying "You received this mandatory email alert to update you on transaction details", various order confirmation emails for things I purchased online, etc.