We are trying to be more systematic in our monitoring and we have not found a simple way to ignore notifications regarding a service that is not yet in production.
What could be the best approach to only send notifications when state is unknown or down, after the host/service check state changed to up at least once?
Generally, You need to send a notification when host/service is down and then recovery notification for the same. Unknown generally happens because of misconfiguration.
You can use icingaweb GUI tool (Director module) to configure notification. On director, dashboard got to notification and create a notification template. Select the transaction type for which you want notification. eg. recovery, problem, downtime etc.