That's an okay use, but in that use you're not attempting to achieving privacy.

Everyone knows you talk to your parents, but code phrases are not a way to get privacy.

It's not for privacy in the way you may be thinking. This was long before cell phones or the internet existed and the conversation would have been over the rotary phone and it is assumed someone is in the house with me that should not be. Goal being police have authorization to kick down the door and assist the person or people that are nutritionally deficient in lead.