Currently the "Zabbix proxy last seen" trigger prototype severity is set as "Warning" in default Zabbix server health template.

https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/app/zabbix_server

A critical condition like an unavailable proxy might go completely unnoticed and not be notified, in most Zabbix installations. 

Please consider

• raising the trigger severity to Average or High
• renaming the event name to "Proxy is not available (last seen > 600 sec)" for better clarity