It is possible, but many people still buy them from their provider with financing or subsidies. That means people shopping for used Pixels who want to unlock the bootloader need to avoid the special Verizon variant which forbids unlocking the bootloader.

This is separate from SIM locking, which forbids use with another carrier. US carriers still do that, but are required to remove the lock after a while if the customer doesn't owe them money.

It's not clear why Verizon insists on permanently locked bootloaders or why Google agrees to it for Verizon when they don't do it on Pixels sold anywhere else.