LG is one of the last remaining OEMs that will allow you to unlock your bootloader as long as your carrier allows them to let you. They have their own developer website which includes a tool to help you along in the process. It looks like they have just added support for the US non carrier version of the LG V20. The specific variant that was added is the LG V20 US996. To unlock your bootloader simply head to the link below, choose your version and follow the instructions. Once your bootloader is unlocked you will be able to flash twrp once available and easily root and rom from there.
via LG