> Anything sent with Signal needs to be treated as published with an unknown delay.

Oddly they have thought of that already, to the point all encryption systems in use in the gov are thought of in these terms.

All that matters are the different assumed times to publication (weeks to years), and then treating the strength of measures involved differently based on what is reasonable for the given use.

If you absolutely need something to never be published then encryption isn't the solution, and nor are computers generally.