This might help you after realizing that you want to install custom ROM again after upgrading to android 11 MIUI but you cannot install android 10 or 11 based ROM and stuck with either boot loop or android rescue party.
WARNING: This might wipe every writable partition. Charge your phone at least 20% or more to prevent from destroying these partitions.
1. Download firmware from here
2. Download full ROM (either recovery or fastboot) from here
3. Save the firmware zip file it in either internal storage or sdcard. Reboot to TWRP afterwards
4. Tap install and find the zip file in your storage and install. Wait for a couple of seconds to get done.
5-a. Dont reboot yet. Go back to install the full ROM zip file. This time it would take long to finish the installation, then format /data. Reboot.
5-b. Reboot to bootloader. Extract the fastboot zip file and run MiFlash. Locate the fastboot folder, choose flashall from lower right side and click start. Wait until it reboots.
6. All done. You successfully Downgraded your Phone and got backed to Stable Build.
Note: I got 2 anti triggered when attempting to downgrade the phone using recovery and fastboot without installing firmware zip file first. RIP me I guess I voided my warranty anyways so doesnt matter as I already applied some insurance for my device.
TL;DR: Installed Recovery will get erased. Please dont skip steps 5. If you did, please do step 5-a but you should boot at fastboot mode and flash the recovery image. Also note that if you dont want to get your bootloader locked, double check if you choose flash_all and not flash_all_and_lock. You should check your config and remove/disable checkpoint. This would lock your bootloader once that it gets done installing.
Thank you very much for the tutorial. I followed your instructions to the letter, the whole process was done without error, however, without positive result, infinite reboots without loading the rom mui 12.08 with android 10.
I had to re-flash a twrp, flash firwmare vendor 12.08, then flash zip miui 11, format data, booting the rom.
In short, I have not been able to go back to Android 10.