Not using inter-cluster encryption for AWS MSK clusters can impact the confidentiality of the data in transit.
Encryption settings are configured with TLS 1.2 by default, however this can be overridden at the time a cluster is created. To learn more, see the AWS documentation (below).
In Terraform -
References:
https://docs.aws.amazon.com/msk/latest/developerguide/msk-encryption.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/msk_cluster#encryption_in_transit