TOTAL:
Zabbix server sets the host availability icon to gray (unknown status) for the corresponding agent interface (Zabbix, SNMP, IMP, JMX) if:
there are no enabled items on the interface (they were removed or disabled);
there are only active Zabbix agent items;
host is disabled;
host is set to be monitored by proxy, a different proxy or by server if it was monitored by proxy;
host is monitored by a proxy that appears to be offline (no updates received from the proxy during the maximum heartbeat interval - 1 hour).
3 Hosts [Zabbix Documentation 5.0]
https://www.zabbix.com/...al/web_interface/frontend_sections/configuration/hosts#unknown_host_status