Telegram has now long had usernames without revealing for private phone number, so it is good to see Signal finally moving in this direction too. It is really not always desirable to have your real phone number being visible in group or community chats.
It is even worse for those who live in risky areas, are experiencing abuse, or who are whistleblowers, etc. Signal’s claim to fame, after all, is privacy.
Unlike Telegram though, Signal seems to be running this test service is a completely separate staging environment, which both parties need to create accounts for.
There is, of course, a downside to going fully over to usernames, and ditching phone numbers altogether. That is that discoverability becomes more difficult as one person really has to invite others to connect with them. Family and friends won’t just find you automatically by your phone number. WhatsApp, Telegram and Signal have been very aware of this and that is why they have had such good networking, as the phone number was always available, and I doubt Signal will be ditching the phone number.
The moment you get onto chat services like XMPP, Wire, Briar, Jami, SimpleX, etc you’ll find it impossible to find anyone you know unless they share their username with you (and even more specifically for SimpleX as each username is unique between contacts).
But with Signal’s new move at least you will be able to protect your phone number from other ad-hoc contacts and group chats.