Warning! Contains SPOILERS for The Flash season 6 premiere.
The Flash season 6 premiere has confirmed the new date that Barry Allen is destined to disappear during Crisis on Infinite Earths. Worse yet, The Monitor has declared that in order for the universe of Earth-1 to be saved, The Flash must die.
Barry Allen's disappearance and possible death during Crisis on Infinite Earths has been teased since the very beginning of The Flash. The final scene of the pilot episode showed the Reverse Flash Eobard Thawne inspecting a newspaper from the future and using it as a means of confirming that his manipulation of the timeline was moving in the direction he had anticipated. The newspaper from April 25, 2024 said that The Flash mysteriously disappeared during some event known as "the Crisis."
The Flash season 5 finale suggested that the events of the season had changed the timeline, accelerating the arrival of the Crisis to sometime in 2019. This was confirmed by the final scene of "Into The Void," the season premiere of The Flash season 6. It was here that the Monitor, the cosmic being who first warned Green Arrow, The Flash and Supergirl of the coming crisis, appeared before Barry Allen and Iris West-Allen and informed them that Barry is now destined to disappear on December 10, 2019 (which, in real life, is when The Flash episode of the "Crisis on Infinite Earths" crossover will air). The Monitor also confirmed that Barry would never return after he disappeared and that in order for their universe to live, he would have to die.
In the original Crisis on Infinite Earths comic book mini-series from 1986, Barry Allen mysteriously disappeared and was presumed dead for over two decades. It was believed that Barry had sacrificed himself to save all of reality, using the vibrations generated by his running to destroy a massive anti-matter cannon meant to destroy all the remaining positive matter universes. However, later stories revealed that Barry had become one with the Speed Force, briefly emerging during Infinite Crisis, before finally returning to reality at the end of the Final Crisis event.
It remains to be seen what the Arrowverse version of Barry Allen's final fate will be. Oliver Queen had bargained to save the lives of The Flash and Supergirl during the The Flash season 6 has opened with a strong running start.