https://arstechnica.com/information-technology/2018/10/new-signal-privacy-feature-removes-sender-id-from-metadata/

Plenty of messaging apps use strong encryption to make it next to impossible for law enforcement officers or other potential adversaries to read communications sent between parties. Often, however, unencrypted metadata—such as the sender, receiver, and time a message is sent—is all the sensitive data an adversary needs. Now, the Signal app is testing a new technique called "sealed sender" that's designed to minimize the metadata that's accessible to its servers.

A beta release announced Monday will send messages that remove most of the plain-text sender information from message headers. It's as if the Signal app was sending a traditional letter through the postal service that still included the "to" address but has left almost all of the "from" address blank.