I did, and that appears to work.

---

$ openstack loadbalancer amphora list; openstack loadbalancer list
+--------------------------------------+--------------------------------------+-----------+--------+-----------------------------------------+--------------+
| id                                   | loadbalancer_id                      | status    | role   | lb_network_ip                           | ha_ip        |
+--------------------------------------+--------------------------------------+-----------+--------+-----------------------------------------+--------------+
| 958bdcb4-a977-4214-bfe9-db9dbaca4c75 | 62e8f78b-6480-4b8c-944b-72554947905c | ALLOCATED | BACKUP | fc00:db05:2f69:60f7:f816:3eff:fe8d:bf9b | 172.16.7.191 |
| 2beb7dc7-5447-470e-abb6-e4616e2a2e44 | 76a20534-9399-4d58-a457-ab0c78a40e74 | ALLOCATED | MASTER | fc00:db05:2f69:60f7:f816:3eff:fe35:71cd | 172.16.7.205 |
| 5d97f6eb-a19c-4e4b-aea0-27fc8e9b8ff1 | 76a20534-9399-4d58-a457-ab0c78a40e74 | ALLOCATED | BACKUP | fc00:db05:2f69:60f7:f816:3eff:fe81:5347 | 172.16.7.205 |
| 5ebd2fe6-e1ee-42cb-8826-77d00b4f72f5 | 62e8f78b-6480-4b8c-944b-72554947905c | ALLOCATED | MASTER | fc00:db05:2f69:60f7:f816:3eff:fe6a:7338 | 172.16.7.191 |
| 3ac0f9c9-f971-492b-ac26-45b539461d37 | None                                 | READY     | None   | fc00:db05:2f69:60f7:f816:3eff:fe82:2c18 | None         |
| 973e075f-8e60-4459-9fa2-0796cabea19d | None                                 | READY     | None   | fc00:db05:2f69:60f7:f816:3eff:fe5c:63ba | None         |
| 9e746b61-80d5-4f8d-9d67-72691937c7ab | None                                 | READY     | None   | fc00:db05:2f69:60f7:f816:3eff:fe82:201b | None         |
+--------------------------------------+--------------------------------------+-----------+--------+-----------------------------------------+--------------+
$ openstack loadbalancer list
+--------------------------------------+---------------------------------------------------------------------------+----------------------------------+--------------+---------------------+----------+
| id                                   | name                                                                      | project_id                       | vip_address  | provisioning_status | provider |
+--------------------------------------+---------------------------------------------------------------------------+----------------------------------+--------------+---------------------+----------+
| 62e8f78b-6480-4b8c-944b-72554947905c | openstack-integrator-aa2c28472727-kubernetes-master                       | 17d25fe98bf5406982f645043c727fe4 | 172.16.7.191 | ACTIVE              | amphora  |
| 76a20534-9399-4d58-a457-ab0c78a40e74 | kube_service_kubernetes-wcovhst42yf6tcdx6qe72supw3apczr1_default_cdk-cats | 17d25fe98bf5406982f645043c727fe4 | 172.16.7.205 | ACTIVE              | amphora  |
+--------------------------------------+---------------------------------------------------------------------------+----------------------------------+--------------+---------------------+----------+

---

As shown above, the loadbalancer come up fine, and with this kubectl output we see that the service gets the correct IP.  (and is reachable)

---

$ kubectl get all
NAME                            READY   STATUS    RESTARTS   AGE
pod/cdk-cats-5fcdf89599-hx7tx   1/1     Running   0          3m20s
pod/cdk-cats-5fcdf89599-lnkdb   1/1     Running   0          3m20s
pod/cdk-cats-5fcdf89599-lsfr9   1/1     Running   0          3m20s
pod/cdk-cats-5fcdf89599-q2nrb   1/1     Running   0          3m20s
pod/cdk-cats-5fcdf89599-rjfhm   1/1     Running   0          3m20s

NAME                 TYPE           CLUSTER-IP       EXTERNAL-IP    PORT(S)        AGE
service/cdk-cats     LoadBalancer   10.152.183.187   172.16.7.205   80:31264/TCP   3m20s
service/kubernetes   ClusterIP      10.152.183.1     <none>         443/TCP        20m

NAME                       READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/cdk-cats   5/5     5            5           3m20s

NAME                                  DESIRED   CURRENT   READY   AGE
replicaset.apps/cdk-cats-5fcdf89599   5         5         5       3m20s

---

Is this a mandatory setting?  or can the charm be configured to detect that no floating-ip-subnet was given and skip the need for this?

The service as configured now:

---

apiVersion: v1
kind: Service
metadata:
  name: cdk-cats
  annotations:
    service.beta.kubernetes.io/openstack-internal-load-balancer: 'true'
spec:
  type: LoadBalancer
  selector:
     app: cdk-cats
  ports:
  - name: cdk-cats
    protocol: TCP
    port: 80
    targetPort: 80

---