As per a post on MIUI Forum, Xiaomi has started rolling out MIUI10 v9.3.21 Global Beta update. The update brings Android 9.0 (Pie) along with March security patch to the device. Note that this is a MIUI Global Nightly Beta update and is currently available for closed beta testers only. You can download the recovery ROM here and stable update is expected soon.
Last week Xiaomi released the list of smartphones eligible for Android Pie/Oreo update that includes both Mi and Redmi range of smartphones. All these smartphones are supposed to get Android Pie/Oreo update sometime before Q2 2019.
List of Xiaomi smartphones getting Android Pie updates apart from Redmi Note 6 Pro are Redmi Note 5 Pro, Redmi 6 Pro and Redmi Y2 (S2).
Android Pie is the ninth biggest release of Android is the successor of the Android Oreo that is currently available in the market. The latest update brings several improvements such as Adaptive Battery, Adaptive Brightness, Slices, App Actions, Overview and few other improvements in the areas like Notifications.