> Seeing all Reddit apps shut down, I can't help but wonder — what is it that made everyone play by the rules? What happened to the adversarial interoperability spirit? What forces people to use their own API keys instead of pretending to be one of the official apps?

The official reddit app uses a seperate, undocumented graphql based api rather than the public REST one they allow for public use. I suspect someone is/will be already using it to emulate the 'old' API soon enough.