Sometimes, when integrating remote data, entities of an entity-type are read-only, i.e. save() and delete() is not implemented. As this is a rather rare use-case, I don't think this warrants dealing with a separate interface or something. Still, I think it would be valuable to have an "official" flag so that at least entities can be marked accordingly and contribs could easily reflect that.
Then, we can document for save() and delete() to throw exceptions if they are invoked for a read-only entity.