This is an adaptation of some chat messages we sent sketching this concept out elsewhere. We've also cross-posted it on our website.

TL;DR: you need a threat model

Sometimes when we're talking, we want ephemerality. Messages that do not last. Messages that cannot be read after some period of time. There are many reasons for this, and I'm not going to explain them, because I'd rather spend energy on how it can or can't be achieved.