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/neptune/latest/userguide/get-started-security.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/neptune_cluster_instance#publicly_accessible