locals {
  dns_names = [
    "b-1.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com",
    "b-2.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com",
    "b-3.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com",
  ]
}
data "external" "find_common_suffix" {
  program = ["bash", "-c", "echo '${join(",", local.dns_names)}' | rev | awk -F, '{print $1}' | rev"]
}
output "longest_common_suffix" {
  value = data.external.find_common_suffix.result
}

# util.py
import os.path as p
import sys
import json
r_dns_names = sys.argv[1][::-1]
r_split_dns_names = r_dns_names.split(",")
print(json.dumps({"output": p.commonprefix(r_split_dns_names)[::-1]}))

locals {
  s = "b-1.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com,b-2.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com,b-3.cluster.abcdef.c2.kafka.us-east-1.amazonaws.com"
}
data "external" "util" {
  program = ["python3","util.py","${local.s}"]
}
output "dns" {
  value = data.external.util.result.output
}

$ terraform apply -auto-approve
...
Outputs:
dns = ".cluster.abcdef.c2.kafka.us-east-1.amazonaws.com"