What's the process for dumping themes that aren't compliant and won't get compliant by the end of the six months (I recall that being the deadline, sorry if it's wrong) they have to do so? They'll just disappear from the repo and anyone with that theme will no longer have the ability to get support or updates?

Things do change, and authors really should keep themselves updated on those changes. With that said, likely most do not and the first inkling they'll have of a problem is when they can't find their theme in the repo any more... but that doesn't help anyone who has downloaded and is using a nice theme they found in the repo that's going to get the boot.