If they find your coded diary, they are just going to torture you until you reveal how to decipher it if they have even a remote reason to suspect you. Everyone thinks they’re a tough guy until they’re waterboarded or they have their fingernails or teeth pulled out with a pair of channel locks. Everyone breaks under torture. The idea that someone can last until they’re killed is a Hollywood fantasy for action films.
The idea that governments are going to expend valuable resources like learned individuals or AI to crack some individual’s code is just Hollywood thinking. If you’re a suspect, the government will monitor you. If you do anything to make them up their suspicion, they will bring you in. They aren’t going to spend untold amounts of money deciphering every suspect’s diaries or writings. They’ll just torture it out of you, either physically(fastest) or mentally.
And let’s just put this out there - if some desperate person fingered you for being whatever, having anything written in code is as good as being guilty to them. You will not return to your previous life. At best you’ll be released and monitored till the government tires of you or it falls. The latter is no guarantee it will end. Just because the government might be replaced doesn’t mean that those taking over will view any more favorably. After all, if you were deemed to be a security threat before, what makes you think they won’t take that to heart. Once deemed a security threat you will always be a security threat, even to those who you may identify with.
I had weird stuff like that start to happen with my BL Touch. Turns out the bracket I’d printed had started to crack in a hard to see spot, so every bed mesh I took was a little different. Am ordering a metal bracket to replace it.