I keep hearing about the need for migration scripts, but all indicates that these do not exist yet (via blocking bug for the next MM3 beta). Is that true?
Yup. It's not hard, if you have the skills necessary to setup a current Mailman 3 beta installation, you almost certainly have the skills to migrate Mailman 2 lists to a Mailman 3 installation (perhaps with consultation here!)
Obviously, we can't require even that level of skill in a public release, so the scripts are a blocker. But such scripts are a non-trivial exercise even when what they implement is "easy": they need documentation, tests, and intelligible error and warning messages.
Is there a way to keep tabs on the progress towards the beta 4 release? looking at launchpad, it looks like development and beta releases were steady up to the last beta. Is there a reason for that?
I'm not sure what you're asking.