Amazon MemoryDB for Redis has supported username/password-based authentication using Access Control Lists since the very beginning. But you can also use IAM-based authentication that allows you to associate IAM users and roles with MemoryDB users so that applications can use IAM credentials to authenticate to the MemoryDB cluster. With this authentication method, you don't need to use a (long-lived) user password. Instead, you use an authentication token generated using AWS Signature Version 4.
There are many benefits to this approach. Instead of managing username and password-based credentials, you can use IAM to centrally manage access to MemoryDB clusters. For client applications running on Amazon EC2, Amazon EKS, AWS Lambda, AWS App Runner, etc., you can inject these credentials (depending on the platform e.g. profile credentials in EC2 and instance role in App Runner) - this provides greater security.