In grepping the sources I see the API is provided and only used in a handful of cases, mainly x86.

On an ARMv8 target is there a power-off use-case for Coreboot?
This is strictly from Coreboot perspective, not Depthcharge.

You're right, there is no globally standardized power off API in coreboot, it's only implemented internally by select chipsets. There is no ARM target that implements something like this at the moment, because they don't really need to.