The Flash disappeared in 2024 during a "crisis" in the newspaper we saw in Season 1. The recording we heard on Legends is from 2056.
Eobard Thawne kept checking back on the 2024 newspaper to make sure Barry disappears then but we still don't know why. Maybe what Eobard is doing in Legends leads to Barry's 2024 disappearance with the Legends needing to head to 2056 to fix it.
On a related note, Bart Allen, Barry's grandson who is also a speedster, hails from 2056.
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