All of the first part of your message constitutes much of the rationale behind Signal Protocol, which is asynchronous, does not assume participants are online at the same time, and does not involve you handing secret keys to a server. But, of course, Signal isn't the only way to accomplish forward secrecy.
Am I "implying" that forward secrecy is required for message security? No, I'm saying it directly.
> All of the first part of your message constitutes much of the rationale behind Signal Protocol
OK, I'll look it up, thanks. Though I suspect I already have (the 3-DH the Wikipedia mentions looks familiar, I suspect it is X3DH). Better than nothing, but not quite perfect.
Am I "implying" that forward secrecy is required for message security? No, I'm saying it directly.