A study released by Google estimates that 1.5% of all logins used across the web are vulnerable to credential stuffing attacks due to being disclosed in data breaches. This number is based off of anonymous login data provided to Google through their Password Checkup extension.
To conduct this study, Google created a breach notification service and an associated Chrome
Password Checkup browser extension that collects anonymous data and hashed logins. When a user logs into a site with the extension installed, the anonymous hash of the login credentials is sent back to Google and checked against 4 billion usernames and passwords that were leaked in data breaches.
If a match was found, a notification like the one below would be shown that warns the user and prompts them to change their password.