The bootloaders of the devices that Google sells in its own store are already unlocked. The only exception are Google phones that are bought from some mobile carriers such as Verizon.
Years later, I tried to install LineageOS and hit the "Flashing Unlock is not allowed" error. It turns out that Google ships warranty-refurbished Pixel 2s with permanently locked bootloaders. No solution, no apology, no indication that it won't happen again:
https://www.google.com/search?q=%22pixel+2%22+%22flashing+un...
Should have opened a small claims lawsuit for the cost of the phone and sue them. It's easy to show the judge "I bought this phone and can use it with any carrier, and when warranty at Verizon was done with it, can no longer use it anywhere other than Verizon"
You simply materially did not get the same (or refurbished) product you brought in.
I have several Android devices where this is not possible, and bringing this requirement to the rest of the Android ecosystem is our "right to repair."
Assuming this remains the case, do not buy Pixels from Verizon.