If an active check is or becomes ZBX_NOTSUPPORTED, it is being correctly reported by the agent and sent to the server! BUT! The server does not change the status of the active check to ZBX_NOTSUPPORTED in the database and the check in the frontend looks like supported.
zabbix_log(LOG_LEVEL_WARNING, "Active parameter [%s] is not supported by agent on host [%s]",
item.key_orig,
item.host_name);
zabbix_syslog("Active parameter [%s] is not supported by agent on host [%s]",
item.key_orig,
item.host_name);
The server only logs the status of the active check.