In no way does Signal prevent conversations from being archived. For all you know, a recipient could be screenshotting all of your messages, and they could even be using the official app when doing so.

If you don't trust your contacts, probably shouldn't be messaging them anything sensitive.

Signal isn't that kind of app. It protects your data in flight, but only has minimal protections after the recipient gets the message. It's a whole other game to protect data at the endpoint. If you can't trust your recipients to protect data, then you shouldn't send them data needing protection. In order to do that you need control over all levels of the device receiving the data, hardware, operating system, file system, and software. Anything else will always leave openings for data at rest at tge destination to be compromised by untrustworthy recipients.

No. Even if they were, the are plenty of ways to capture the messages.

It's their computing and their devices, not yours.

Well yes, anyone can compile its own version of Signal and use it and it will work as long as there aren't some major changes to its communication protocol

The direct answer to your question is: verification of the security of the platform that the other party is using is outside of the scope of the Signal protocol. Anything you send to the other party can be taken off of their device. Signal only concerns itself with securing the message over the network and making it hard for an adversary with network dominance to build a social graph. It doesn't protect from all SIGINT.

Additionally, since the server is open source and the protocol is open an publicly documented, it is completely possible to build your own Signal client and give it whatever capabilities that you'd like.

There are several open source packages available that allow you to interface with Signal without using the official Signal client:

https://github.com/AsamK/signal-cli

https://gitlab.com/signald/signald (also, https://signald.org/articles/clients/ )

Any signal conversation can be archived

What is the threat model where this matters? You have to trust the recipient with Signal. The only one I can think of is the case where your recipient is using a compromised fork and is unaware. In this case, talking about the tool and checking with them about what they are using is really the only countermeasure.

I'm out of the loop what's going on?