It is locked. There is a switch like with the Chromebooks, but to reach it you have to disassemble it. OTOH rooting and running custom kernels works. Not ideal but it works for some use-cases.

edit: The Pixel C runs on Coreboot of course.

Yep, but has anyone successfully installed Linux (aside from ChromeOS, obviously)?

UEFI is the standard firmware these days, at least for ARM servers.