I broke a data pin on my current phone so it only has the stock ROM with F-Droid.
My new phone is a Xiaomi that I don't use enough for Xiaomi to allow me to unlock the bootloader, so also stock ROM with F-Droid for that one until I actually start using it. Once I get the bootloader unlocked, I'll put Graphene OS on it.
Basicly you may be able to reboot into fastboot and unlock the bootloader without changing any settings in the os. It really just depends on the device.