(I used to work at Coursera) It's actually up to the professors to decide - some of them leave them up, some close the classes. Sometimes they'll close because they want to improve the videos for next time or prevent super simple cheating in the next session, for example. We came up with proposals for how we could leave all classes open while also alleviating professor concerns, but I don't know how far those are along. There's a lot on their plate, as you can imagine!