And what were the dominant terrestrial species at said time? Would there even be any? I have zero idea of what’s the expected survival rate of an iPhone in the fossil record, but I like to imagine a giant sloth stepped on one once and we are none the wiser.
Alternative title:
“Assuming the timeline is stable and time travel is possible, but no evidence is ever found until its invention, what would be the earliest acceptable time period for time-travel tourism, and the latest cut-off point no one would be allowed to go to?”
They need to go back to when earth was very hot ball of molten metal. Anything more recent than that means there is a non zero chance that the smart phone or parts of it would remain intact. There is also a chance of turning into a fossil (the phone gets buried into some mud, the mud dries and turn into a rock, eventually the phone dissolve and disappear leaving behind its print in the rock
You could go check out Earth 1.0 before the collision that created the moon and melted the Earth in the process.