I agree that automated CodingKey synthesis implies treating the keys as keys rather than 'data' (which is sort of how I see the PreformattedCodingKey: that the keys are data that should be left alone).
I'll add a few other people to this thread: @drexin (since you are reviewing my PR related to SE-0320 which also regards Codable), @tomerd (since you initiated the broader discussion about improvements to Serialization in Swift).