The App Store pages say for both the Navigon & Garmin apps that they will be removed May 14th, but will be supported for two more years.
Of course if the apps break due to an iOS update, it will be immediate.
That being said, my main of the Navigon use was as a just in case when driving in areas without cell service, or driving in Canada & other countries without using expensive roaming data. Does anyone have any suggestions of offline driving map apps to replace these discontinued apps?