Yeah, Signal is more than encrypted messaging it’s a metadata harvesting platform. It collects phone numbers of its users, which can be used to identify people making it a data collection tool that resides on a central server in the US. By cross-referencing these identities with data from other companies like Google or Meta, the government can create a comprehensive picture of people’s connections and affiliations.

This allows identifying people of interest and building detailed graphs of their relationships. Signal may seem like an innocuous messaging app on the surface, but it cold easily play a crucial role in government data collection efforts.

Strictly speaking, the social graph harvesting portion would be under the Google umbrella, as, IIRC, Signal relies on Google Play Services for delivering messages to recipients. Signal's sealed sender and "allow sealed sender from anyone" options go part way to addressing this problem, but last I checked, neither of those options are enabled by default.

However, sealed sender on its own isn't helpful for preventing build-up of social graphs. Under normal circumstances, Google Play Services knows the IP address of the sending and receiving device, regardless of whether or not sealed sender is enabled. And we already know, thanks to Snowden, that the feds have been vacuuming up all of Google's data for over a decade now. Under normal circumstances, Google/the feds/the NSA can make very educated guesses about who is talking to who.

In order to avoid a build-up of social graphs, you need both the sealed sender feature and an anonymity overlay network, to make the IP addresses gathered not be tied back to the endpoints. You can do this. There is the Orbot app for Android which you can install, and have it route Signal app traffic through the Tor network, meaning that Google Play Services will see a sealed sender envelope emanating from the Tor Network, and have no (easy) way of linking that envelope back to a particular sender device.

Under this regime, the most Google/the feds/the NSA can accumulate is that different users receive messages from unknown people at particular times (and if you're willing to sacrifice low latency with something like the I2P network, then even the particular times go away). If Signal were to go all in on having client-side spam protection, then that too would add a layer of plausible deniability to recipients; any particular message received could well be spam. Hell, spam practically becomes a feature of the network at that point, muddying the social graph waters further.

That Signal has

1. Not made sealed sender and "allow sealed sender from anyone" the default, and
2. Not incorporated anonymizing overlay routing via tor (or some other network like I2P) into the app itself, and
3. Is still in operation in the heart of the U.S. empire

tells me that the Feds/the NSA are content with the current status quo. They get to know the vast, vast majority of who is talking (privately) to who, in practically real time, along with copious details on the endpoint devices, should they deem tailored access operations/TAO a necessary addition to their surveillance to fully compromise the endpoints and get message info as well as metadata. And the handful of people that jump through the hoops of

1. Enabling sealed sender
2. Enabling "allow sealed sender from anyone"
3. Routing app traffic over an anonymizing overlay network (and ideally having their recipients also do so)

can instead be marked for more intensive human intelligence operations as needed.

Finally, the requirement of a phone number makes the Fed's/the NSA's job much easier for getting an initial "fix" on recipients that they catch via attempts to surveil the anonymizing overlay network (as we know the NSA tries to). If they get even one envelope, they know which phone company to go knocking on to get info on where that number is, who it belongs to, etc.

This too can be subverted by getting burner SIMs, but that is a difficult task. A task that could be obviated if Signal instead allowed anonymous sign-ups to its network.

That Signal has pushed back hard on every attempt to remove the need for a phone number tells me that they have already been told by the Feds/the NSA that that is a red line, and that, should they drop that requirement, Signal's days of being a cushy non-profit for petite bourgeois San Francisco cypherpunks would quickly come to an end.