Because this is end-to-end encryption, what's transmitted is only arguably "the message", we can't - and there is every reason to think never will be able to - decrypt it without the symmetric key and only the sender and recipient have the key.

So from an information theoretic point of view, arguably no, the message was never stored anywhere else, even though Signal is indeed a store-and-retrieve arrangement. The encrypted message was stored briefly by Signal, but only its sender and intended recipient could decrypt it.

The keys are ephemeral, so if you have exactly bit-for-bit copies of encrypted Signal messages I sent when I was arranging to play Red Dead Redemption 2 with friends, nobody knows how to decrypt those. I can't decrypt them, the people who received them can't decrypt it, even though we saw them at the time, and even though you have the encrypted messages and even if you have our phones, the keys are gone so that's that.