How to register Nine in Galaxy S10 with Android Pie instead of Unmonitored App.
In Nougat and Oreo, the Device Care app had a section that said "Unmonitored Apps." You could add apps for this section that you did not want to ever be closed or out to sleep to save power.
In Pie, this section was removed, but the whitelist still exists. Here's what happened, Decide Care battery saving features somewhat overlap Dose. Rather than maintain two different "never optimize these apps for battery usage," they instead pull from the Doze's Optimize Battery Use whitelist. Find this number going to Settings->Search->Optimize battery->Tap, tap again. Adding an app here, prevents you from adding that same app to the "Always Sleeping" section of Device Care, thus demonstrating that this list is the new home for the "Unmonitored Apps" section.
A new whitelist was also created, focused on background apps. Device Care now offers by default to Auto Care every day, this cleans up temp files and closes background apps. Apps aren't actually force closed though, instead, their UI activity is closed, and the service (if one exists) stays running. A whitelist was created for this Auto Care feature. Found in Device Care->Memory->Menu->Apps that aren't checked.