If you want to downgrade the OS of your LG V40 from Android 9.0 Pie to Android Oreo 8.1 First, You need to check the Anti-rollback version of your device, and then You need to identify the Anti-roll back Version of the kdz firmware you want to flash.
How to check the Anti-rollback version of your LG V40
From your Phone Dialer,
You will see the software version in detail, scroll download to see the Anti-rollback version.
Some LG V40 model is not Qfused, So the anti-rollback version says “Your Phone is not Qfused” –
How to check the anti-rollback version of the KDZ firmware you want to flash
Step 1, You need to use WindowsLGFirmwareExtract tool to extract aboot_xxx.bin or laf_xxx.bin from a kdz file
Step 2, Use a Winhex to open aboot.bin extracted
in the above screenshot, the anti-rollback version is 00 or V0
in the below screenshot, the anti-rollback version of a kdz firmware is 03 or V3
For example, Your Phone’s current Anti-rollback version is 03, Flashing a kdz firmware with ARB version less than 03 will hard brick your phone.
You are free to flash anything kdz firmware with ARB version equal or higher than the ARB version of your LG V40 smartphone.
Need more help, please visit XDA forum here.