I was reading the https://github.com/nre-learning/nre-resources/blob/master/NRE%20Labs%20-%20OSS%20NA.pdf and noticed the page on Build Lessons in the Browser. A good alternative to Cloud9 is TheiaIDE https://www.theia-ide.org/. The benefit of Theia IDE is it is not tied to Amazon’s infrastructure, it can be deployed anywhere.
I am currently working on a lesson for Theia IDE, which could be used as an intro lesson for Building New Lessons.
In addition there is gitpod.io which is Theia Hosted.