MIUI is Xiaomis Android skin and a long time ago, the company used to develop its UI for non-Xiaomi devices. These days, Xiaomi doesnt do that anymore, but it does allow its devices to be unlocked and flashed as the customer pleases. The company even offers full ROM files on its community site.
MIUI 10, based on Android 9.0 Pie has been successfully ported to both the OnePlus 6 and OnePlus 6T unofficially, of course. Given the nature of different Android skin frameworks, not everything will work from the get go.
The post is based on Xiaomis 8.11.23 build of Android Pie and in order to install, youll need to have version 0B8 on your OnePlus 6(T) or newer. Of course, youll need some general ROM flashing know-how beyond this point. After all, youre flashing software that wasnt intended for a OnePlus device.
The developer reports that mostly everything is working including the notch and automatic brightness. What still needs work is NFC, the Flashlight toggle, the Stock MIUI camera, Mi Accounts, and the Auto Rotate toggle will cause your settings to freeze.
The port is more of a proof-of-concept more than anything. Those who flash ROMs on their devices know that certain apps like Netflix and Google Pay wont work due to security concerns of having an unlocked bootloader, which exposes the personal data should the device fall into the wrong hands.
You will need to have your bootloader unlocked and the TWRP custom recovery installed before you can proceed with this project. If you dont know what any of this is, we wouldnt recommend you try flashing ROMs to your daily driver.