When I was a teenager my bff and I used to have a notebook that we passed back and forth writing letters to each other in code so no one would know the identity of the boys we liked.
But as an adult I've never coded anything in my journal other than using abbreviations, but that's more for my convenience. I always think that if someone reads something in my diary that they don't like, oh well, they shouldn't have read it in the first place. LOL.