Would invites be a solution? Anyone can sign up if they provide a number, otherwise you need an invite from someone with a number linked. It would clump the identity/legitimacy for all invitees into origin number, but still allow disparate accounts.
they did not "make it required", Signal was just never developed to support anything else for username/registration. Which is what they have now almost corrected.
It's wording then. Making something required sounds like an artificial limitation whereas implementing support for usernames requires a lot of work, it's not like they commented out a couple of lines on purpose.
What is required at the moment is any phone number, not your phone number. You can use a phone booth even.