For example, on WhatsApp, use the whole 25 character limit for profile name. Examples:
Bob Moved To Signal.org
Alice MovedTo Signal.org
CharlieMovedTo Signal.org
Say Signal.org, not Signal, because they won’t know it’s an app.
Use your about section too. Same on Discord, Steam, Instagram, etc.
The Signal server is centralized, but it is also still open source under the AGPL license: https://github.com/signalapp/Signal-Server