To reduce the number of users who mods their devices, HTC decided to add a TEMPERED flag to device with an unlocked bootloader, you can see it when you are in bootloader mode, the flag found at the top. For some folks the flag quite disturbing because it seems we have been destroyed our own devices, though we never did it. For M8 users, now you can be glad since we can remove the TAMPERED flag from HTC One M8 bootloader, this should work all models Verizon, AT&T, Sprint, T-Mobile and various other.
One of the requirements that you must meet before removing TEMPERED from the bootloader is gaining S-OFF, for those who still S-ON you will never be able to get rid of the flag from your device. Moreover, gaining S-OFF is not that hard, not like HTC smartphones a few years ago, now we can simply use the one-click exploit like Firewater.
To be able to eliminate TEMPERED flag from HTC One M8 is not too difficult since we only need to type a few commands through the command line prompt. However, it’s not too easy for novice users, because we have to get familiar with adb or fastboot command.