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?”
Assuming the time line is stable I would say any time in before a global catastrophic event such as the K-T dinosaur extinction event since any meddling there will be likely erased by the destruction. The latest cut-off point should be when the first true humans appeared, around 2-6 million years ago.
However, how does your novel account for not disrupting the timeline? Is the model a parallel universe, i.e. going back in time creates a new reality branch? If such is the case, then there are ethical issues for the putative time traveller.