Amazon RDS Instances can be created independent of clusters, yet they can still maintain numerous databases despite being isolated. Allowing unrestricted, public access to cloud services could open an application up to external attack. Disallowing this access is typically considered best practice.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/infrastructure-security.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance#publicly_accessible