https://www.zabbix.com/documentation/3.0/manual/web_interface/maintenance_mode

The message at the end of documentation say:
"
The following screen will be displayed when trying to access the Zabbix frontend while in maintenance mode. The screen is refreshed every 30 seconds in order to return to a normal state without user intervention when the maintenance is over.
"
But the refered screen only will apear for users without IP address defined on $ZBX_GUI_ACCESS_IP_RANGE parameter