For reference, I have an OG Pixel. Bootloader is unlocked, but everything else is stock Android after flashing the March image just this morning.

Android Headlines / Google News / Google Pixel / Version 1.4 of BootUnlocker Will Lock and Unlock the Nexus 5's Bootloader Without Losing Your Data ...

Unlocking the bootloader and rooting your Galaxy Nexus is really only half the battle. There may come a time when you need need to lock everything back up, including that bootloader. Thanks to the ...

To install TWRP, however, you do need an unlocked bootloader first, which is why this particular unofficial version of TWRP for the Galaxy S8 Snapdragon models is rather significant.

The bootloader, in almost any computing environment (not just Android), is executable code that runs before the operating system loads and starts to run.