How does AWS notify AWS Clients about security incidents or suspected data breaches

Hello.

Security incidents that occur on the AWS side are notified on AWS Health Dashboard, etc.
https://docs.aws.amazon.com/awssupport/latest/user/incident-response.html

Communication is made by the Security Alternative Contact registered in the AWS Account.

For more details:

• Update the alternate security contact across your AWS accounts for timely security notifications - https://aws.amazon.com/blogs/security/update-the-alternate-security-contact-across-your-aws-accounts-for-timely-security-notifications/