Exactly. How hard can it be? It would make it way more secure for almost no effort. I would use Signal if it did not require a phone number or an e-mail address (whichever it requires).

I'm okay with it requiring an e-mail address, that's easier to be universally accessible, as long as your e-mail provider is sane and doesn't require SMS confirmation to login.

Phone numbers, for most people, only work within a certain part of the world and aren't easily accessible worldwide.

The reason I am not okay with it is that it is usually tied to your person in one way or another. Have you tried getting an e-mail while using Tor? Next to impossible. Plus, it is unnecessary really. You can use nicknames or say, a generated UUID of some sort, with perhaps a QR code for easy copying.

crating a new yandex email account works nicely via tor for me

I will check. I would say that it works... for now. :P It is not something we should rely on, I think.