It's correct to say "it's [ it is] written"

如果用 “it”为主语，这个动词的第三人称单数通常用被动语态。，只有一次我看到有人用那个动词的第三人称单数形式。那是在古波斯诗人莪默•伽亚谟写的《鲁拜集》中： The moving finger writes, and having writ, Moves on; nor all thy piety nor wit Shall lure it back to cancel half a line, Nor all thy tears wash out a word of it. (第51 章)

If you want to say that something has been written down, you must say "written" because it was done in the past. If you want it to be written down, You can say "I want this to be written down." Or "Please write this down for me," Here is the strange thing about 'to be written'... You are saying, though it is not written down already, in the future you want it to have already happened or finished (the act of being written.) If you want to see more about this, it is called a future perfect tense.

You can't say "it writes" unless you're talking in the third person about something writing. You could say "look at that robot! It writes!" But you can't use it to mean the same thing as "it's written." We rarely ever say "It's written." We'd usually say something like "It says..." But it depends on the context.