From reddit :
Hey everyone!
As of today, Dark Web Monitoring is available to all paying users of Proton Mail.
Proton alerts you if an online service you’re registered for has had a data breach or has been hacked: https://proton.me/blog/dark-web-monitoring
To get started, open the Security Center in the Proton Mail web app, or go to Settings>Security and privacy: https://proton.me/support/dark-web-monitoring
We look forward to your feedback!
The Proton Team
Big bummer indeed!