As a kid I thought if i write English in a mirrored way i.e. right to left, & mirrored, it would be my code script, nobody else can read it. People read it very easily.

That’s how Leonardo wrote, because he was left-handed, so it was more practical — it preventing smudging the writing with the wrist.

(Except Leonardo didn’t write in English.)

Not just smudging. The entire mechanical act of writing (and everything) is optimized for left handed operation being the mirror image of right-handed operation, since left is mirror image of right.

That was usually in draft mode, since the left-to-right and right-to-left lines wouldn't quite line up perfectly.