Signal Introduces Usernames, Allowing Users to Keep Their Phone Numbers Private
Setting a new username requires account holders to provide two or more numbers at the end of it (e.g., axolotl.99) in an effort to keep them "egalitarian and minimize spoofing." Usernames can be changed any number of times, but it's worth noting that they are not logins or handles.
It's an opt-in feature
users can control who can find them by their numbers using another setting, restricting people from messaging them even if they are in possession of the phone numbers.
Both these options can be toggled via the following steps -
Settings > Privacy > Phone Number > Who Can See My Number > Everybody / Nobody
Settings > Privacy > Phone Number > Who Can Find Me By Number > Everybody / Nobody