When Youtube refers to Vitess as being RPC-based, they are not referring to Sun RPC (callrpc), but rather to the generic design pattern of exposing service calls over the network. In particular, Vitess makes services callable [-] using either BSON or JSON serialisation over HTTP CONNECT calls.

[-] http://code.google.com/p/vitess/source/browse/go/rpcwrap/

If you look at the original RPC work by Bruce Nelson [1], it's pretty clear that there's no strict definition of it. I think most would argue that SOAP would be included, which is still pretty common.

1: http://nd.edu/~dthain/courses/cse598z/fall2004/papers/birrel...

That's ONC RPC. Not the only implementation. XDR is still used in places, though you're right that it isn't widely used.