Is the Boardloader of the Trezor Model T unmodifiable, undeletable, or unupdatable? If I install an unofficial Bootloader, will the device display a prominent warning?
yes
If you install an unofficial bootloader, the device will RSOD. You will need to restore to an official bootloader by flashing it on a SD card.
See also here Is there any risk in updating the bootloader of Trezor Model T? - #2 by matejcik