> Protocol Buffers

Well, REST is architecture. Protocol Buffers are (de/)serialization.

I've tried it before (& although it has the properties I like[language agnostic schema definition..]), it was finicky to get working with of one the biggest IDEs out there(IntelliJ IDEA)

Also, it doesn't support Kotlin yet.(Java interop only option)

The only place I can see Protocol Buffers being useful is inside Google..cos it was a pretty crappy dev experience.

And I would be pretty confident in saying that not many Googlers/ex-Googlers outside Google use it. Cos if they did, somebody would make the dev experience much much better.