Previously, during a device unenrollment, all applications were removed that where pushed/installed via Microsoft Intune by default. This is not always handy, for example take the Microsoft Office applications. You can add multiple accounts within these applications and if you are using Microsoft Outlook for your work mail and also for your private mail. You want Microsoft Outlook to stay on your mobile device after an unenrollment.
With the August 12, 2019 update of Microsoft Intune it is now possible to control the app uninstall behavior at device unenrollment for iOS devices.
To configure this, login the Microsoft Azure portal and navigate to: Microsoft Intune > Client apps > Apps
Open the application you want to configure, in this case I will select Microsoft Outlook.
Open the Assignment page and edit the assignment. With this new update, you now have the option to uninstall the application on device removal or not.