Scalable Server relies on Protobuf for its client protocol. The Scalable Server has been updated to use Protobuf 3 internally, and for the protocol definition.
Clients using Protobuf 2 are still fully compatible with the new Protobuf 3 Scalable Server, as both are wire-format compatible. However it is heavily recommended to migrate clients to Protobuf 3, by using the latest protocol definition files (.proto) available in the Scalable Server FAQ. These changes will soon be fully integrated with the Unity SDK.
You can learn more about protobuf in general by checking the official documentation.
The Asmodee Digital team