[gepoch]

Signal stores messages on their servers until they're delivered at which point they're purged.

Additionally, Signal's encryption scheme gives their messages the "forward secrecy" property which means that acquiring key material at some point in the future does not allow you to decrypt any previous messages. Any encrypted messages that they could provide would be useless.

For more, check out their really interesting doc on the double ratchet algorithm that they use!:

https://signal.org/docs/specifications/doubleratchet/