Can't find the blog post on the Proton website anymore, but here's an official response from the Proton support team on Reddit: https://www.reddit.com/r/ProtonMail/comments/1cmcre5/comment/l33oxua/
Sending plain text notifications is impossible for Proton btw, since they don't hold the encryption keys to your email inbox and can't read your emails themselves, and thus can't send you a notification with the email subject and body.
I'm trying to help out wherever I can :)