You no longer have to give out your phone number if you want to be reachable by Signal Messenger. That move was overdue and it came in true Signal fashion.
You can change that username at any time and you can choose what you share on your profile. I chose @vowe.01 as my username, but I do not accept messages from people who do not disclose their identity.
Signal is my preferred messenger, but I can also be reached by iMessage with my email address. Sorry, no WhatsApp.