I suspect that the reason for WhatsApp, Signal, Google & co's insistence on requiring a legacy phone number for account creation is that linking it to the user's profile is the easiest and cheapest way to associate it to an individual person for legal compliance, linking it to a national jurisdiction.

It is more like phone based 1FA at this point. My Google account cannot be recovered at all even though I know the password and still control the recovery e-mail because at one point I made the mistake of adding a (now inaccessible) phone number.

we need the equivalent of ipv6 privacy extensions for phone numbers

You don't need to use a phone for 2FA, at least not in my experience (never connected mine). It can be with a hardware key instead (yubikey, etc). They don't make it very clear in the UI process, but it definitely has worked for me.

That said, not a fan of forcing 2FA if people don't want it.