Warning: The following contains SPOILERS for Batwoman's "Take Your Choice," The Flash's "Death of the Speed Force" and Supergirl's "The Bottle Episode."
The episodes following immediately after the Arrowverse Crisis on Infinite Earths event suggest a horrifying extinction event took place entirely off-camera; worse yet, most of the heroes seem entirely unaware that thousands, perhaps millions, of people died in the wake of Earth-Prime's creation.
The final chapter of Crisis on Infinite Earths established Batwoman, usually the most grounded of the Arrowverse series, that was the first show to address the question of whether or not anyone from the pre-Crisis Earths managed to escape to Earth-Prime on their own.
For reasons that were never explained, The Flash episode "Death of the Speed Force," declaring that any doppelgängers who wound up on Earth-Prime "would all be dead by now due to neurological degeneration,"when Nash Wells asked about the possibility of pre-Crisis doppelgängers existing elsewhere on Earth-Prime.
This declaration stood in stark contrast to a whimsical scene in "The Bottle Episode;" the first episode of an evil alternate version of Winn Schott and multiple versions of Brainiac-5. Yet, nothing was said on Supergirl about what happened to all of the other doppelgängers after "The Bottle Episode."
It beggars belief that out of all the Arrowverse, only Kate Kane would have an experience like this, or that Al's Dive Bar was the only nexus to suddenly spring up on Earth-Prime. Indeed, Cisco Ramon took a leave of absence from STAR Labs to travel the globe and chart all the anomalies between the pre-Crisis and post-Crisis Earths, because there were so many that needed ing. Additionally, Gideon, the AI Barry Allen created (in the future), calculated that there were 3.725 trillion changes, when tasked with calculating how many things had changed in the episode "Grodd Friended Me."
Logically, this means that there should be thousands, perhaps millions, of doppelgängers who came to Earth-Prime from other worlds in the pre-Crisis Arrowverse. However many there were, it also logically means that all of them died slow, agonizing deaths shortly after their arrival, along with their Earth-Prime counterparts. This may explain why Cisco was so quick to snap at Nash Wells when he was questioned about being certain that there was no way any doppelgängers survived; Cisco was likely a first-hand witness to the greatest pandemic in human history, and was utterly powerless to stop it.