You do understand that a modern or post-modern society will have their own tools to detect radiation? We don't need to optimize for them, we need to optimize for those people who haven't developed that far.
A skull and crossbones is a pretty universal symbol of death.
Oh yeah? Have you seen any research demonstrating that people unfamiliar with that iconography associate it universally with death? Or are you assuming they would, since you've seen it associated with death your whole life due to pirate media etc?
Your solution is barely above "why don't they just write NUCLEAR STUFF, DANGER?"
Some mythology that speaks of ancient ancestors who created magical rocks that can melt your flesh off at a distance so that they could turn the daylight on inside isn't likely to deter anyone but the most gullible and least inquisitive.
Yeah, not like millions of people have been following such rules for thousands of years. Much better to use iconography they probably won't know or understand.
It also assumes that such a made-up religion would survive longer than any extant languages and scientific knowledge, which is absurd.
We LITERALLY have religions that survived their languages & scientific knowledge dying off, and most of our modern religious ideas are simply remixes of previous ones. You couldn't be more wrong if you tried.