There's no technical limitation, but read this (https://news.ycombinator.com/item?id=22357948 https://news.ycombinator.com/item?id=22357948) comment to understa

There's no technical limitation, but read this (https://news.ycombinator.com/item?id=22357948 https://news.ycombinator.com/item?id=22357948) comment to understand why Signal did what they did. Your SIM card stores your contact list across devices. With usernames, your phone's contact list doesn't work anymore. You need a persistence, i.e. cloud backups, to store the contact list first. This is in the works (https://signal.org/blog/secure-value-recovery/ https://signal.org/blog/secure-value-recovery/). Expect to see user names once that's done. The nice thing is the persistence can also remember identity keys, fingerprint and their verification statuses, and even group membership status -- so it'll mean much more convenient use once it's ready.