I've seen this done with the on-the-wire protocol of scientific measuring equipment. It's hardly just for RPC (which HTTP increasingly resembles anyway).
The point is, the ability to "type" a protocol is irrelevant to how modern distributed software gets developed. Maybe it mattered in the days when comms were at 300 baud, machines had kilobytes or perhaps megabytes of core, and the Mark I eyeball was the best way to debug machine-to-machine comms, but these days we have tools that can decipher binary wire protocols for us. Performance and adaptability are far more important now than human readability. That war has been lost.
The point is, the ability to "type" a protocol is irrelevant to how modern distributed software gets developed. Maybe it mattered in the days when comms were at 300 baud, machines had kilobytes or perhaps megabytes of core, and the Mark I eyeball was the best way to debug machine-to-machine comms, but these days we have tools that can decipher binary wire protocols for us. Performance and adaptability are far more important now than human readability. That war has been lost.