That's probably why they killed it—it gave the end user too much control over content they consume and how they consume it. And of course you can't tell what they actually read or when....
Isn't it possible to simply check when the request to the RSS feed is made? It won't tell you which posts the user read, or even if the user read them at the time of the request (I'm thinking caches), but it could at least tie IP or some other drive-by info to a rough idea of readership.