Proxmox API response for storage resources includes a status attribute which can be "available" or "unknown".

The official template does not include an item prototype to retrieve this information, so any issue on storage availabilty cannot be detected by Zabbix.

    {
      "plugintype": "pbs",
      "node": "pve7",
      "content": "backup",
      "storage": "ONG-PBS01",
      "status": "unknown",
      "type": "storage",
      "id": "storage/pve7/ONG-PBS01",
      "shared": 1
    }

Moreover, when a storage is unavailable it does not include "disk" and "maxdisk" attributes for space monitoring, then JSONPath preprocessing goes silently unsopported.