Thought experiment, suppose in a remote part of the world, someone comes across a site with this exact message tomorrow and tweets it. What is the most likely scenario:
A) People heed the message and avoid the area.
B) The place is overwhelmed with people trying to find some hidden treasure or have alien encounters, etc.
I would argue that scenario B, is much, much more likely.
For better or worse, curiosity especially about forbidden stuff has been a hallmark of humanity, and likely will be so into the future. All this sign will do is encourage people to come and try to find the treasure and the mysterious source of power.
Have all your pictograms try and communicate that some color = you're going to die, as well as "If you move much beyond this point, you're going to die _fast_."
Here's the important, albeit macabre part: After about 100 meters towards the center of the exclusion zone, you need to store enough radioactive material, preferably near Geiger obelisks, that going there will kill you _very_ quickly, for 1000's of years. Quicker the better. People need to associate the deaths with the area!
I assert that when 50% of people get cancer a month after entering a dump zone, it's hard to associate those illnesses with the cursed area until a lot of people are doomed. If 3 people go somewhere and none return, it becomes part of local lore.
Interesting idea, but people will gather radioactive material to prepare the suicidal attack. The cursed area therefore needs to be able to kill people without an ability to move the curse out of it by itself.
You'd also need to design signage/language to last and transcend those same 10K years to indicate what that constant "blip blip blip" means.
Not to mention that it's typically an annoying ego-wank, since the people asserting it never seem to include themselves in the purported devolution...
Bury it in such a way that any hypothetical future primitive neo-civilisation can't detect it and can't dig it up - for better or for worse.
Instead of drawing attention to the dump of strange rocks that make neolithic humans sick, why not make it so unobtrusive that any enterprising future miners don't discover the waste dump in the first place?
We have the technology to survey geological formations for exploitable minerals. Why not instead survey for both the lack of exploitable minerals AND a long-term stable environment, then bury the small volume of high-reactivity waste that we've produced so far in a tiny shaft with a tortuous access route?
It's a lot easier to get humans to ignore something for 10,000 years than to make them pay attention to a warning sign.
There has to be treasure down here look at the ways they are trying to prevent us from finding it. We've got to dig this up. - future me
Also why do all these people think a) that we are around in 10.000 years or b) we are tribal people at this point?
Watch "Into Eternity", it talks a lot about that. The plan for Onkalo (Finland) is to use both : the caves will be sealed and hidden, but they will also place warnings inside.
Too much left-brain in most tries. The modern 'biohazard' symbol is not scary... nor is the ISO radiation sign. 'Skull and bones' is moreso.
Many old Hollywood films kept it simple with warning messages posted around territories ... heads on pikes, hanging skulls, that sort of thing. They say 'Enter here and you will likely die' pretty effectively.
Even George Pal's morlocks were scarier than that ISO sign. Looks like an ad for sunblock.
Also, the biohazard symbol was designed to be memorable but meaningless so people could be educated.
Maybe the primitive people back then were getting cooked to death from standing under the sun too long? Good thing we have clothes nowadays.
Really makes you empathize with religious writers trying to communicate often second hand divine truths that people thousands of years later will have to figure out without the understanding the original prophet had.
Of course, the real problem with this endeavor is that human nature is going to be that explorers ignorant of The Old World will happily ignore any signs saying "Danger! Keep Out! We Really Mean It! This Will Kill You (Slowly)!" looking for treasure. That's exactly what happened with the pyramids--they were looted within a few centuries of being constructed. If humanity regresses to a point that they don't understand radiation poisoning, they are not going to be stopped by even the clearest explanation of what will befall them because they won't believe it.
A machine that can reanimate the dead perhaps? Energy that can turn bones into a living person?
Why not say "nuclear" or "radiation"? It seems very unlikely that the entire species will completely forget about the concept of nuclear radiation.
That it isn't fiction is what gives it salience, and the requisite paternalistic 'talking down' to a pre-technical future aspect provides emotional and/or moralistic flavor, if you like that sort of thing.
Eventually, it turns into bike-shedding scary apocalyptic artworks.
We don't sit around moping that Marie Curie & co died doing research, everyone involved gets recognised for helping to usher in a new age of scientific progress.
In one scene, a character is ruminating on what the mysterious "Fallout" might be, that their ancestors were trying so desperately to take shelter from. He pictured it as a towering monster, that prowled around hunting for people to eat.
It's a fun thought to imagine what people will think of these warning signs in 5000 years, assuming there are still people left!
e.g. "There's something deadly buried here? Neat, I'll dig it up and throw it at my enemies!"
Perhaps they'd consider it covered by saying "nothing of value is here"...?