I think that is a misrecollection. I spent a day or two on the original MITx platform hacking to make sure page close events *were* being logged accurately. Guaranteeing that the event was properly fired before the page closed was a pain in the butt, but I eventually got it to work, at least in the browsers I could test with. I heard from several people at edX that the MITx system was broken since we did not have a heartbeat, and so could not log time spent on task, and spent a while trying to squash that rumor (apparently, unsuccessfully). 

It's possible someone broke/removed/etc. them in edX at some point, but if so, I was unaware of it. There are many things I'm unaware of, however, and I haven't looked at/for them in a while.