schweizer
While the phone number requirement is problematic, the rest of that post is pretty ill informed.
Unless you make it onto a US (or Five Eyes to a lesser extent) NatSec targeting list where the US federal government is willing to expend serious effort going after you, the US is actually about the most privacy friendly jurisdiction around.
There are zero restrictions on what kinds and degree of encryption are legal in the US.
Any attempt to go after/target someone based on speech or political beliefs is going to run headlong into the First Amendment.
US courts are notoriously bad about localizing foreign subpoenas for data. Unless the foreign LEO's can get the FBI on board with the investigation and to apply for the subpoena, it basically goes nowhere.
If law enforcement serves a subpoena on Signal they receive 1) associated phone number, 2) date the Signal account was established, 3) date of last Signal login.
Law enforcement can clone your phone number and thus swipe the associated Signal account and thus receive any future Signal messages sent to you but they can't see past messages or contacts.
If your threat model involves serious, directed, effort from the US NatSec establishment then you should default to basically everything electronic being compromised.