An open protocol can mandate indeed, but that is still in the realm of pinky promise security. A better design for a privacy-friendly chat protocol is to not write a lot of stuff on a lot of different remote servers when that's not necessary IMHO. One of matrix's selling points is to be censorship-proof though; in that case copying stuff as much as possible makes a lot more sense.

>pinky promise security

You are right, though I still prefer "weak feature" as a term :) There's enough value in such things. Cryptography crowd is concentrated on omnipotent Eve breaking ciphers, and that wrench from xkcd, but I dare to claim that majority of both commercial and private leaks happen just because well-intentioned users don't have enough capacity to keep track of all the things, and proverbially think twice. Features like "unsend", or timed deletion are indeed laughable on their purely technical merits, but do wonders saving users from grave mistakes anyway.