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
I know it’s their entire business model, but presumably they’re discovering the data for every proton email address and not just the paid ones. It would presumably cost them nothing to provide this feature to all users, I wonder what their reasoning was for making it a premium feature.