Off the record was specifically intended to be just that: an online discussion with no evidence of what was said or even that it took place. Forcing both partie

Off the record was specifically intended to be just that: an online discussion with no evidence of what was said or even that it took place. Forcing both parties to be online at the same time is consistent with that. When the discussion is over then it is done and gone. The protocol used by Signal Messenger adds on the pre-key stuff as sort of a bag on the side to enable asynchronous messaging. It's basically a place to dump a bunch of keys that will later be forgotten as part of a forward secrecy scheme. So stuff sticks around until the recipient goes online and sees the messages. Then the keys are forgotten. At some point the client has to dump in some more keys to keep up the stock.