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Yes they are they've been like that since fallout NV
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Fallout NV companions could die in survival or if you shot the to death
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No they couldn't
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I distinctly remember accidentally killing Boone with grenades a few times, and them getting tore up by deathclaws in survival. Maybe you never tried that difficulty but they could and did die. But usually only in survival.
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Yh most likely in hardcore
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I'm pretty sure they all are