Warning: this article contains spoilers for the Arrow Season 7 finale and The Flash Season 5 finale!
Both Arrow and The Flash just wrapped up their latest seasons. One thing both finales had in common (besides an emphasis on the heroic legacies of Oliver Queen and Barry Allen) is that they teased the upcoming “Crisis on Infinite Earths” crossover. In the case of Arrow, Ollie’s happy retirement was interrupted by the Monitor, who informed him the time had come to honor the bargain they struck in “Elseworlds.” On The Flash, the infamous newspaper headline declaring “Flash Missing, Vanishes in Crisis” suddenly changed. Recent alternations to the timeline have moved up the Crisis from 2024 to 2019.
Source: IGN.com Are The Flash and Arrow's Season Finales Teasing a More Interconnected Arrowverse?