(First of all, sorry for bad english)
We know that “the formal end of the Roman Empire on the West in AD 476 thus corresponds with the time in which the Empire and the title Emperor no longer had value.” (wiki)
Centuries in the future. An alien is studying the falls of the “modern human era”, occured in the Gregorian year 20XX. There was no a “deus ex machina” single and short event like a total nuclear war, an alien invasion, a cosmic disaster etc.
But, for example, an economy recession period + other contributing factors (regional nuclear conflict and/or a pandemic and/or a bad programmed AI and/or a small volcanic winter etc) (What a great period to be alive, uh?). Modern human society (slowly) collapsed and never recovered. Humans are still alive but they never returned to a "technological society".
In that hypothetical future alien book of exo-history, what could be some formal events that declared the end of the “modern human era”?
- UN shutdown?
- The last day that the US President had some kind of power?
Let's say that this alien race has a vision of the "society" similar to us, then let's exclude too peculiar events ("the day that Facebook was shutdown", "the day that the CERN doors was closed", "the last chinese restaurant in NY" etc).