I swear it's been a clause in their phones since they were called Nexus instead of Pixel where you breach the warranty when you unlock the bootloader. I never bother anymore but when I used to swap the Android versions myself I recall running into something saying as much.
> breach the warranty when you unlock the bootloader
Under the Magnuson-Moss Warranty Act in the USA, activities such as unlocking the bootloader, launching a service menu, removing a sticker or opening a case cannot by themselves void a device's warranty.
If I understand correctly, in order for the warranty to be voided, it is the manufacturer who has to prove that what you did to the device has indeed damaged it or made it otherwise unsuitable for further warranty service.
Unlocking the bootloader is a reversible action. The phone might implement some one-way unlocking mechanisms though. For example, a fuse which needs to be blown. Or some encryption chip whose private key needs to be erased while a new valid private key can only be generated by the manufacturer. Then it would be a process that is irreversible for a regular customer.
That being said, undertaking such activity would only result in some control mechanism being triggered and some software flag being set. It might be permanent, similar to you scratching the case of your phone. But that does not mean it makes the phone unfit for warranty service.
The phone's functionality would remain unaffected.
It would be on the manufacturer to prove that the presence of a flag indicating an open bootloader is in some way detrimental to the device's functionality.
