I have a really split opinion about this. On the one hand I've seen a lot of really hateful stuff lobbed around on sites like Twitter, and I suspect that linking accounts to phone numbers would dramatically reduce that. On the other hand, I'm not sure I want Twitter (et. al.) knowing my phone number.
I think that at this point FB has proven that people will be nasty regardless of how not-anonymous they are.
As such I'm doubtful this will change anything about peoples behavior.
It's also quite scary how nonchalant many people here are arguing for this to stop "propaganda" which these days seems to be as easily defined as "Anything that doesn't conform with a Western/US-centric narrative".
Because I have yet to see one of these "propaganda ban waves" be reasoned with anything but "Russia/Iranian/Chinese propaganda" like that's the only kind of "propaganda" that exists [0].
As such I consider these "propaganda bans" just another exercise in propaganda [1].
This is bit of hypothetical, but does some sort of pki-like scheme exist that would allow me to hold a certificate (of sorts) from an authority that I could use to prove myself to an service, but also simultaneously would not leak any information about me to that service? Similarly service a and service b should not be able to link accounts behind my back. Sounds like a interesting crypto problem
There'd need to be some central authority (like you said) doing the certificate issuing that verifies that you are, indeed, a human, and that your a unique human that they haven't already given a certificate to.
Chances are they'd want more than a phone number; probably a photo ID or something as well, else their value proposition isn't very strong.
I'm not sure if that's better for privacy and safety than many services asking for just a phone number (which I can generate a semi-throwaway Google Voice number for).
Seems like it would make a lot of sense for them to offer both options; either would discourage bots, and you'd give people the choice whether to go the free route or the privacy-conscious route