IIRC Signal did get a "knock at the door" and, rightly, had almost nothing to give the government - mostly because of how they designed their architecture, but admittedly partially because of the metadata they chose not to store.

Lavabit's situation was different because they were a "choose not to look" solution, not a "can't look" solution.

That said, Signal is centralized, so it's not like they are particularly robust against that sort of thing, just that other than getting Signal shut down, any strong-arm tactics are unlikely to result in very useful information from OWS.