> Why would you want this?
Because I have repeatedly dug out useful information from chats, days, weeks, months, or years later.
> If something important comes up, like an address or recipe, copy/paste it into your notes app.
You're assuming that 1) you know what's important at the time, rather than realizing later, and 2) you want to take the time, at the time, to figure out somewhere to file it.
> I used to be a message-hoarder too, but I recently realized it was all utterly useless
That's your choice, but that doesn't make it the right choice for everyone. Your preferences are not universal. (And descriptions like "hoarder" deride the choices of others.)
> Signal isn't email.
People advocate using Signal in place of email, for security.
I cannot advocate Signal to anyone I know until it learns to treat user data as incredibly valuable and irreplaceable.
If people want to mark their messages as transient, or even mark all their messages as transient, so be it; that's their choice. But if a message is not marked as transient, it must be possible to securely and easily preserve that message for longer than the lifetime of any one device.