langchain/chains/conversational_retrieval/base.py/ChatVectorDBChain/raise_deprecation
class ChatVectorDBChain:    def raise_deprecation(cls, values: Dict) -> Dict:
        warnings.warn(
            "`ChatVectorDBChain` is deprecated - "
            "please use `from langchain.chains import ConversationalRetrievalChain`"
        )
        return values