Signal’s mission and sole focus is private communication. For years, Signal has kept your messages private, your profile information (like your name and profile photo) private, your contacts private, and your groups private – among much else. Now we’re taking that one step further, by making your...
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Each version of the Signal app expires after about 90 days, after which people on the older version will need to update to the latest version of Signal. This means that in about 90 days, your phone number privacy settings will be honored by everyone using an official Signal app.
Also from the article (clicking the dots in the text)
Also, the Signal servers refuse to talk to certain software at arbitrary times.
They only expect the official software to be used despite purporting to allow forking the client software.