I mean, surely, all of the more advanced aspects of ancient societies were biodegradable?
We tend to assume that global society is the most advanced it has ever been, but what if that's just our arrogant ego talking, based on a mirage? It should be obvious that only the most primitive metal and stone symbols (pyramids) would have the dumb longevity to survive 1000s of years. Even so, they've given us so many hints about their mathematical abilities.
What if ancient societies got so cynical about "dem young inbred next generation" that, in multiple acts of desperation, they inscribed a whole bunch of runes for us, dumbing it down of course, like a kind of mental seed bank??
Ancient satellite and Internet technology? Biodegradable.
Ancient nuclear power? Safe enough not to leave any long-term evidence on the planet. The
lack of evidence for (ancient damage, isotopes scarring the environment, etc.) is not
evidence of lack (Logic 101). If anything, any "first time ever" occurrences of scarring should be regarded as positive evidence that we have gone downhill.
Ancient mining and excavation of energy and minerals out of the earth? Advanced quantum teleportation, unlike the primitive metal scoops that we use today.
That is all.