Signal has spam issues even with the phone number requirement, as I've experienced lately (though nothing on the scale of Twitter). I dread to think what the spam would be like without the requirement of a phone number.
At least now you can solve the existing spam problem if you want by disallowing people from using your number to message you in the privacy settings and randomizing your username after anyone new adds you - that way your username is like a one time password to add you, kind of like what lots of people here wish existed for phone calls.
Just like you haven’t received any communication from anyone about any topic other than talking about Matrix. It’s not that Matrix has a magic formula, it’s that a fraction of a fraction a percent of people care even an iota about it.
They could collect a small amount in cryptocurrency to prove user is not a spammer. Telegram tried this but the price for not providing a phone number was too high. Does it mean knowing user's number is so valuable?
It strikes me as hopelessly naive to think that keeping a personal phone number private is the only reason a user would want to be able to sign up for a service completely anonymously. The question is not whether knowing a user's number is worth $X, the question is whether _anonymous access to your platform_ is worth $X; a question that applies equally to both innocent good-faith users and to spammers/phishers/etc. If your platform is actually worth anything, $X is not going to be a small amount.
And yet many people seem to earnestly believe that a tiny token fee will be enough to deter spam, despite clear evidence to the contrary (see for instance how Twitter's "verification" fee has completely failed to stop bots from overrunning the platform, many of which proudly display their blue checks).
