The question I would ask is: given the list of capabilities presented in this post, is there even any difference between Riot and e-mail? Or are you reinventing the wheel?
Riot is most comparable to Slack or Discord. It has chat rooms. It supports voice, image posts, file transfer, etc. It stores conversation history. You can private message people.
Matrix is a generalized protocol for decentralized and federated communications; it's agnostic to the application layer provided by Riot. Something Matrix doesn't have, but is on the issue backlog, is support for email-esque thread contexts. [0]
End-to-end encryption. Even if you encrypt email with PGP, which no one has come up with a satisfactorily easy interface for, it leaks a lot of metadata. Riot gives essentially the same privacy guarantees as Signal, but with email-like federation.
If two users PGP email to each other, anyone who's monitoring either mail server or the network in between can tell A) when they are talking, B) who they are talking to, C) what the subject of the email is, and D) how big the email is.
With signal this is much harder, for instance google/apple do not know who you are chatting to. They also don't handle the encrypted message transport/delivery.
None of the email headers are protected in any way for a PGP-encrypted email. All the same metadata is that collected from plaintext email is still available on "encrypted" email. You literally can only protect the body of the email. In surveillance, that is often the least interesting or valuable piece of information.
As mentioned signal doesn't hide contact discovery. But it does a pretty good job of hiding who you are chatting to from everyone but OWS.
OWS received a Grand jury subpoena and was only able to produce "the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user's connectivity to the Signal service.".
Certainly a NSL might compel OWS to add additional logging (and not talk about it). With that they could tell who messaged who, when the message was sent, and how big the message was.
> Certainly a NSL might compel OWS to add additional logging
NSLs cannot be used for that. They're a legal tool that can be used to extract certain types of information (such as subscriber information and maybe a little bit of transactional information) that a service provider already has stored on their servers [0]. However, they cannot be used to force a service provider to write and deploy code.
>As mentioned signal doesn't hide contact discovery. But it does a pretty good job of hiding who you are chatting to from everyone but OWS.
Anything that does TLS to connect to a single server can do that. Heck, if you do email to a single email server using secure IMAP and secure SMTP then PGP no longer leaks metadata.
Signal does not solve the meta data problem either. Discovery is an open problem [0]. Signal messages leak the recipient, which is the most important meta data. You would have to use Tor/Onion routing, which is inefficient.
Matrix is a generalized protocol for decentralized and federated communications; it's agnostic to the application layer provided by Riot. Something Matrix doesn't have, but is on the issue backlog, is support for email-esque thread contexts. [0]
[0] https://github.com/matrix-org/matrix-doc/issues/492