In an imaginary and fictional context; I'd imagine a 'time traveller' would have some top level Govt' to Govt' communication. So we'd probably not even know.
Then again keeping in that context it would possibly depend on the evolution of systematic governing changes.
IE; how far ahead the regressed had 'travelled'
I could imagine someone from the hypethetical year say... 2030 would have more to worry about than one from the year 5450 - the further advanced would probably lend to confidence issues.
Then again once you hit the exact year that such tech would be possible on a frequent scale, there would be much confusion in advancement vs regression.
I'd be more interested in the opposite time travel possibility, IE; instead of regressing, progressive 'travel'.
If we believe that the sciences behind building blocks on a quantum level are aware of observation; then the act itself may cancel itself out in a type of 'universal' protection trigger.
Meaning, we already have, but we well never have known' - how's that for a kicker?
lol... interesting topic. On a side note that RMT may be able to relate to...it's too bad the term 'evolution' has been limited for popular use to a transmutation of species reference and not more towards an understanding of personal 'energy/energies'.
Does anyone know if we can make energy from absolute nothing? Or if the creation of it of itself, has to be drawn from related sources on a quantum lvl?
Then again, I'll probably get the response that absolute nothing is a debatable topic in and of itself?