I'm not the one who build this method I just wanna share the only way I found to downgrade this phone to android stock. I'm not responsible for any brick or bad use you may have with your phone.
* Bootloader Unlocked
* Motorola Drivers Installed on your PC.
* USB Debugging Enabled on your Device
* A Moto G8+ XT2019-2 Compatible with this firmware version
(All your information and data will be erased from your device. Please make a backup before proceed).
Your can download the firmware build without bloatware or pre-installed apps from my one drive storage here
After decompressing the file your can read the "DOHA" .txt to see all details of this android 9 build. Also you can right click to "Flash ROM" and "edit" to check all command line on the main file.
The last step its connect your Moto G8+ on fastboot mode to your PC and double click to "Flash ROM.bat" a CMD window automatically open to flash your device. DO NOT CLOSE THE WINDOW. Wait until finish and the device will be downgrade to android 9 stock.
If everything was fine the CMD window will automatically close and the device will be restart.
* With this firmware version will NOT work some moto gestures.
* The system goes more smoothly and apps open more faster.
* Battery life improved a lot more compared to android 10
* You can still receive OTA updates