I've hated this about Signal since first beginning to use it. Forcing everyone to use a phone number makes it so much harder to have a partially anonymous way to communicate.
I can't help but feel this was intentional to prevent use by those wishing to remain anonymous. I don't believe they'll ever actually do this.
If they could have they would have done so already imo. Perhaps they are fighting whatever restrictions force them to do this. We can only hope.
My belief is that various governments prevent Signal from allowing anonymous registrations and will continue to prevent them from allowing it.
I expect that what they will allow is something like "you can now register with a gmail account". That is the same restriction since you need a phone number to register for a gmail account these days.