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.
What does that mean? REST is a style while protobufs are a serialisation format: a REST architecture can use JSON, protobufs, Thrift, whatever.