During MDL-63466, the get_conversation_messages method has been added to the message/externallib.php for adding support to conversation messages. 

For now, this method is based on the data_for_messagearea_messages, which is the one used from the UI. However, the message/externallib.php file contains also the get_messages method, which is quite similar. For this reason, we should add some extra parameters and logic to the get_conversation_messages method, for being able to deprecate also the get_messages method.