How to specify path.data on startup of 6.3.1

No, never saw this.

Here is the full configMap I'm using for your reference:

apiVersion: v1
kind: ConfigMap
metadata:
  name: elasticsearch-config
  namespace: backend
data:
  elasticsearch.yml: |
    cluster:
      name: ${CLUSTER_NAME:elasticsearch-default}

    node:
      master: ${NODE_MASTER:true}
      data: ${NODE_DATA:true}
      name: ${NODE_NAME}
      ingest: ${NODE_INGEST:true}
      max_local_storage_nodes: ${MAX_LOCAL_STORAGE_NODES:1}

    processors: ${PROCESSORS:1}

    network.host: ${NETWORK_HOST:_site_}

    path:
      data: ${DATA_PATH:"/usr/share/elasticsearch/data"}
      repo: ${REPO_LOCATIONS:[]}

    bootstrap:
      memory_lock: ${MEMORY_LOCK:false}

    http:
      enabled: ${HTTP_ENABLE:true}
      compression: true
      cors:
        enabled: true
        allow-origin: "*"

    discovery:
      zen:
        ping.unicast.hosts: ${DISCOVERY_SERVICE:elasticsearch-discovery}
        minimum_master_nodes: ${NUMBER_OF_MASTERS:1}

    xpack:
      license.self_generated.type: basic