Perhaps you're right that they couldn't be compelled by law to reveal it, then! However, I can still find people on Signal using their phone number, by design. If they can do that, surely there is sufficient information, and appropriate means, for US state-side signals intelligence to do so, too. I don't think Signal self-hosts their infrastructure, so it wouldn't be much of a challenge considering it's a priority target.

Now, whether FBI and friends would be determined to use PII obtained in this way to that end—is a point of contention, but why take the chance?

Better yet, don't expose your PII to third parties in the first place.

Yeah it should be technically feasible to do "eventually" but it's non trivial. I linked a bunch of their blogs on how they harden contact discovery, etc. And of course you can turn contact discovery off entirely in the settings.

Settings > Privacy > Phone Number > Who can find me by number > Nobody

https://news.ycombinator.com/item?id=46786794