A hobby project from about 5 years ago https://cmdchallenge.com . There is zero monetization and the only reason I keep it running is that it has been pretty low maintenance and doesnt cost anything to run (one small $5 vm). There seems to be some small groups who use it as a learning tool. I don't think it is a great way to learn shell fundamentals by any stretch but the low barrier makes it a bit more accessible than other tutorials.