Major Description
AuthInfo, even if no longer used, needs to remain in the thrift build, in case there are thrift code generation bugs identified and fixed in future 1.5.x releases. The way AuthInfo was deprecated (the class was removed from the thrift generation, and a deprecation tag was added to the class itself) means that any such fixes will not be applied to AuthInfo.
Instead, if this object is not intended to be used in the public API (which it isn't, because it's for internal serialization for RPC calls, like all the thrift objects in core), the places where it is exposed to the public API need to be deprecated, not the AuthInfo class itself.