*Sigh* Will have to find or create a mod to correct that. It was certainly a core aspect that all companion NPCs in Fallout (and Fallout 2) could be killed if the PC was careless or unlucky with them.
I don't see the value in companion NPCs that are immune to death (except to use them as shields to exploit their invulnerability); and I don't see the PC ever caring about them if they are never at risk, and cannot be taken away. The most intense times in the Fallout series, that I recall were those fights where my NPCs barely survived, and did so because of luck and careful tactics by the PC. It looks like that won't happen anymore under Bethesda's stewardship of the IP.