[jrm4]

But again, I think it's already reasonably solved, by the solution we have now, for email.

Again, a universe of difference between e.g. Twitter (where anything can be revoked and they have ultimate power) and the problems with e.g Gmail (you take some care, you just start over, you start your own server.)

As I tell my students -- from a technical POV, mails from gmail.com and mails from jrm4.com (my personal domain) are just about equal on the Email protocol. This is pretty remarkable and a pretty good system already.