It is hard to respond to you in good faith because it does not feel like you want to engage that way. Maybe I am misunderstanding your statement. So I will engage in good faith once more (reminding you that this is a HN rule).
Anonymity is nowhere near as simple as not requiring PII during registration. Even from the provider side. We can agree that it is difficult to maintain anonymity from the user side. But choices made by the provider can make these things impossible. Let's suppose Signal gives me one username that I can use and I must use that continuously. What do I choose? If I use "godelski" then I've made a permanent connection between that identity and my real life identity. After all, I do talk to friends, family, and coworkers on Siganl. If I choose another name and share that name through this name, then those two are linked together. All of this is information leakage and highly valuable to OSINT people. I'm sure someone here has significantly more experience than me and would be happy to expand upon this too. But it is extremely naive to believe/claim that anonymity is provided simply by removing the phone number. It is far more complicated than that. Hopefully you now know that.