Steps to reproduce:

1. Changes in configuration...
2. Want to trigger a reload
3. try systemctl reload zabbix-server

Result:

Failed to reload zabbix-server.service: Job type reload is not applicable for unit zabbix-server.service.

Expected:
It should just work.

/etc/systemd/system/zabbix-server.service.d/reload.conf is the general way to support it:

[Service]
ExecReload=/usr/sbin/zabbix_server -c $CONFFILE -R config_cache_reload

/etc/systemd/system/zabbix-proxy.service.d/reload.conf:

[Service]
ExecReload=/usr/sbin/zabbix_proxy -c $CONFFILE -R config_cache_reload