Yes, indeed not supported yet, one of the reasons ...

# spring

[sdeleuze]

Yes, indeed not supported yet, one of the reasons is that I am not sure yet how async/non-blocking Protobuf API is. We currently support Smile (Binary JSON) and asap Jackson CBOR supports non-blocking API we will add support for CBOR which would be my #1 choice for binary http APIs, see https://jira.spring.io/browse/SPR-15961