One of the biggest advantages of Android over other smartphone operating systems is the ability to obtain root privileges and change around the system software as you please, even to the extent of wiping out the manufacturer's software entirely and replacing it with something like Cyanogenmod or stock Android. This procedure depends on the device's bootloader, which is the basic layer between the core hardware and system software, being unlocked. These days, however, more and more devices, such as the Samsung Galaxy S7, are shipping out with locked bootloaders. T-Mobile is actually working with Samsung on unlocking their Galaxy S7 variants' bootloaders, leading users to question whether the LG G5 would have an unlockable bootloader when it dropped. According to XDA Developers forum member spanky34, who bravely took the plunge and voided his warranty for the good of the community soon after getting the phone, the bootloader is unlockable and can even be relocked.
The unlock procedure is very similar to the one that would be used for a Nexus device; one simply checks "OEM Unlocking" in the developer options, boots the phone up into fastboot mode, plugs it into a computer with the right drivers and tools installed, then uses the Android Debug Bridge to send the command, "fastboot oem unlock". This will unlock the bootloader, allowing unsigned code to run on the phone through the fastboot interface. This means that things like custom recovery menus and other deep, system-level tweaks can now be developed for the new flagship.
Beyond the bootloader unlocking, of course, there are other obstacles standing between LG G5 users and the wild world of custom ROMs. Among the bootloader variables shown in the console output from spanky34's phone was a somewhat worrying option that read "(bootloader) secure: yes". As with any low-level tweak, this is not fool-proof. If a user accidentally disables the OEM unlocking option in their settings, for example, while the bootloader is unlocked, it will be impossible to unlock again. If LG releases a tool to flash officially signed ROMs via fastboot later on, this could be remedied, but for now, users should exercise caution. The phone is still brand new, so it will take a while for a custom recovery and custom ROMs to be developed, but the way is apparently open.