Apparently Signal is working on identifiers different from a user's phone number, but it is not clear how many people will actually take advantage of this feature.
Often this answer is accompanied by pure sarcasm where if you are concerned about this feature, you are told that Signal is not for you and "you can go play at being a spy and sharing a secret decoder ring with your friends", as these people regard PGP to be. I wish those Signal advocates could lay off the sarcasm, it just makes the project look bad.
The uncertainty as to how many would use it is likely why it’s been back burnered for so long, but it shouldn’t impact effectiveness. I realize that you may not have been implying it would though.
[1] https://www.signal.org/blog/private-contact-discovery/
Of course, this feature is Android only as iOS doesn’t allow default alternatives.