According to @autoprime – a Developer at XDA forum, It is possible to re-lock bootloader on the H850 variants.
@wolfgart, a LG G5 H850 user also confirmed that it is allowed to lock bootloader again on LG G5 H850.
How to relock the bootloader:
Issue this command: fastboot oem lock
bootloader should now be locked… and the yellow warning sign when booting should also be gone.
*Confirmed works on H850 model with V10c Stock Firmware.
We are not sure if LG will allow the re-lock of bootloader on LG G5 H850 model in future. Future Software Update may Block the re-lock of bootloader.