Some background here: https://www.class-central.com/report/coursera-old-platform-shutdown-download-courses/
Here's the email Coursera sent:
###Save course materials for some courses by June 30
Dear , We wanted to inform you of an update to our technology platform that will affect access to some courses you previously joined. In 2014, Coursera began developing a new technology platform to improve your learning experience, and to allow courses to run more frequently. The majority of our courses are now offered on the new platform.