Correct me if I’m wrong, but from what I’m reading online, Proton doesn’t “market” or “advertise” discounted plans beyond what you’ll save by purchasing a longer plan for their VPN. They do have a page on coupons here: https://proton.me/support/coupons

And based on what it says, as well as other tweets and reddit comments I’ve seen from their official accounts, they grant these coupons on a case by case basis. So forgive me for being skeptical, but I don’t believe it was false advertising, nor do I believe it was presented as a plan to begin with.

Unless you have a screenshot that shows them blatantly telling you it was a plan, I think you probably misread what they said, and that it “suddenly a coupon and not a plan anymore” isn’t actually truthful. Though of course, you could very well be right, and I might just be skeptical for no reason