Is this true for all carriers? Or just Verizon? Several Reddit threads say that it's just Verizon. T-Mobile users report being able to bootloader unlock after getting their phones carrier unlocked by T-Mobile.
OEM/carrier locking tends to only happen in the US. Many carriers permit unlocking after the contract has concluded, but Verizon has always blocked it. Used devices also run the risk of being improperly unlocked and making OEM unlocking remain unavailable. (Used devices may also be Verizon devices.)