If you want to be able to easily root your phone and flash custom roms you are going to need an unlocked bootloader. Over time OEMs have decided it is less advantageous for them to allow you to be able to remove whatever bloat ware apps you like installing your own custom software and have made it increasingly difficult to unlock the bootloader on their devices. LG has continued to be developer friendly allowing their devices to be bootloader unlocked.
LG just released the bootloader unlock method for the European LG G6. You can now use their official bootloader unlock tool. Know that if you do use the tool to unlock your bootloader your device’s warranty is void. The bootloader process apparently can not be reversed so it’s not like you could just relock it to get your warranty back. The fact that the European bootloader will be unlockable may mean there is hope for root on US variants as well. You can head to the link below to access the bootloader unlock tool which will walk you through the process step by step.