One example that was created by a community member here (and is not affiliated with Let's Encrypt) is
It uses a slightly different approach than the Let's Encrypt expiration notices. The latter are based on a history of all certificates ever issued by Let's Encrypt, while the former are based on actively monitoring whatever is being used by a particular specified site. So for example if you issued a Let's Encrypt cert for example.com and example.net, and then you completely stopped using example.net and renewed the cert only for example.com,
-
the Let's Encrypt warnings would say that you had failed to renew your example.net cert, because the CA knows that you once had a particular cert configuration and you no longer do, while
-
the Let's Monitor warning would say that everything was fine, if you had only signed up to monitor example.com, because at least that site still has a valid cert.