Interesting, you can see who among your circles has quit G+ via these steps:
Profile > Your Circles, switch sort to “Not yet using Google+”
For me, people who never joined G+ have emails on those card while people who used to be on G+ have just names. At least one person changed their name to “Johnny McDeleteme” before removing their account, which was clever IMHO. I might have been able to figure out who that was by analyzing my circles, but I didn’t bother before removing from all circles.