Lethe appears in Republic 621a, where souls camp in the plain of Oblivion beside the river Ameles, whose water no vessel can hold, and drink before being carried off to birth. The personification of Lethe as Forgetfulness goes back to Hesiod's Theogony 227, and the word itself simply means concealment or oblivion, the opposite of aletheia, truth as un-forgetting.
Orphic gold tablets buried with initiates in Thessaly, Crete, and southern Italy instruct the dead: you will find two springs, do not drink from the first, it is Forgetfulness; drink from the spring of Memory, and tell the guardians who you are. These fourth-century BCE grave goods read like boarding instructions for keeping your identity through death.
Plato's reincarnation includes an engineered amnesia that severs personal continuity between lives. Readers suspicious of the afterlife machinery ask the obvious question: a system that requires forgetting before each round is not accountable to anyone in it.
If forgetting is the standard procedure and remembering is the rebel's path, which tradition had the true map of the afterlife, and why did the other one get a state funeral?