Carrier-sold Pixels generally don't have "OEM-unlockable" bootloaders.
Your best bet for now is to buy a new Pixel direct from Google, or a used one from eBay that the seller advertises as already having GrapheneOS on it (or otherwise guarantees that the bootloader is unlockable). These ones are worth a lot more than the ones that can only run Google/carrier Android.
https://grapheneos.org/install/web#prerequisites
I own two GrapheneOS Pixel 7 units, which should get any Google blob security updates (which GrapheneOS incorporates) through October 2027, and GrapheneOS may still support it with source updates after that. So in a year or so, I might get the GrapheneOS Motorola if it's available, or a later Pixel. (I never buy these new, since I don't want to carry a several hundred dollar phone when a 2 gen old one is still great, thanks to GrapheneOS.)
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.)
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.)