A canary goes something like "This website has not received or acted on any government orders to disclose or modify or remove material." When they ever do, then they remove that notice. The government enforcement usually includes a gag order prohibiting the target from saying that they're under orders, so the intent is that you can infer government gag pressure by the canary having been removed. Wikileaks used to have such a notice and no longer does, so we assume government enforcement is why.

I'm not sure what the cryptographic part has to do with anything. I'd guess it was signed in a way that you can verify the government itself didn't tamper with the notice.

Their signing key expired 2007 source is them:

https://wikileaks.org/wiki/WikiLeaks_talk:PGP_Keys

Wikileaks information was trustworthy and accurate. It may still be that the information prior to their canary expiring is okay, but anything released after can't be trusted.

I'd assume that once the canary died whichever actors compromised them scrubbed it.

It's been over a decade now, but I do have a machine somewhere with evidence.

Some nerd bigger than me here certainly has evidence available in a dropbox or somewhere accessible. I don't.