Not understood at the protocol level, not visible as such in the client.
> Your own client or provider can store history. you also don't own someone else's history - a good feature imo.
It's a nightmare in practice. Catching someone up on a conversation ends up being a mess of forwarding multiple overlapping but partially distinct emails and then constantly having to resend messages as different people get dropped in different replies. Exiting such a conversation is even harder.
> A preference for individuals to decide.
The cost is much bigger than the benefit.
> What's a "real" identity? my email address is my identity.
There's no authentication. You can't associate yourself with multiple organisations except by using multiple addresses, and then there's no way to reflect that those are somehow the same.
no shared concept of a group/channel/...
no shared history support
no shared concept of a thread
no consistency about how previous replies are referred to (e.g. different quoting styles)
no real identity support
...