Morrowind is a great example of a game that didn’t protect quest critical characters.

If you killed one, you would get the message:

“With this character's death, the thread of prophecy is severed. Restore a saved game to restore the weave of fate, or persist in the doomed world you have created."

But not completely doomed actually, because the creators buried a back-door method of victory into the game. It requires following obscure clues hidden in books, killing the very god that otherwise would have helped you, and makeshift rigging up an artifact in a way that harms you, but the much more difficult hidden backpath to victory does exist, and bypasses every other plot element:

https://en.uesp.net/wiki/Morrowind:Yagrum_Bagarn_and_Wraithguard