Steps to reproduce:
1. Create a new project
2. Switch to android platform
3. Go to Player Settings > Resolution and Presentation > Aspect Ratio > Custom and set it to 3
4. Deploy to device

Expected behaviour: Unity rendered fullscreen
Actual behaviour: A black bar in notch area

Reproduced on: 2018.2.10f1, 2018.3.0b3, 2019.1.0a3 (0a3d74)

Reproduced on:
VLNQA00173, Xiaomi Redmi 6Pro (Redmi 6 Pro), Android 8.1.0, CPU: Snapdragon 625 (MSM8953), GPU: Adreno (TM) 506

Not reproduced on:
VLNQA00025, Xiaomi Mi 5s (MI 5s), Android 6.0.1, CPU: Snapdragon 820 MSM8996, GPU: Adreno (TM) 530
VLNQA00166, Huawei P20 (EML-L29), Android 8.1.0, CPU: HiSilicon Kirin 970, GPU: Mali-G72

By design: letterbox(black bars) is applied by the android on all apps that don't explicitly define that they should be rendered around the notch. Unfortunately means to control notch behaviour on devices running Android <9 is entirely dependent on manufacturer, so there is no unified solution for that.

So far there are several options for controlling notch behaviour on Android <9:
Huawei: add to manifest <meta-data android:name="android.notch_support" android:value="true"/>
Xiaomi: add to manifest <meta-data android:name="notch.config" android:value="portrait|landscape"/>
Others: should react to "layoutInDisplayCutoutMode" activity theme attribute, but that's not guaranteed