Sure, if they’re granted access to the channel. But that access would come from users of the channel, not the service itself, and if the service doesn’t store the keys (i.e. you need at least one user online to get access), the service can’t really help the feds.

And whether to provide access to history for new users can absolutely be a setting on the channel. I’m just saying that having the messages only on the clients doesn’t preclude sharing those messages with a newcomer.