"Yes. Incorrectly implementing the HTTP spec is widespread though"
Huh??? First of all, that's a pretty serious statement to be making with no evidence to support it. Plus, that is no excuse for any server that is NOT compliant.
The very fact that nginx decides to create HTTP status codes, willy-nilly, for its own use shows at least the suspicion that strict compliance is not a priority for them. Thankfully, it is for other web servers.
From the few days I ever did API design (as a trainee, not even employee yet), idempotence is one of the first things you encounter when looking for general design and it definitely came into view for me. Then again I'm not the average student, but still.
People are writing custom HTTP application servers and making PUT do anything and everything. Idempotence doesn't usually enter the picture.