Button Codes

This strange cheat fades the player who entered it to a near-invisible state. This state lasts for as long as the player lives. If they die, the code must be entered again.

Unlike the first four cheats, this one does not display a message when entered.

This rendering mode makes the screen pure white except for certain edges — edges where the foreground and background reside at different z-depths. It seems to be some kind of debug mode related to edge detection and maybe anti-aliasing.

This is what Rare had to say about Line Mode on their old website:

"Q: Is the 24th cheat a line mode/pen and ink mode?"

"A: No. The cheats were devised to try and make the game more fun in both the single and multi player games. How a line mode would have done this eluded us, so we kept it out. It might possibly be activated using a GameShark or similar device, and though it is a useful debugging tool, as a gameplay feature it is a bit lacking."

(Source)

To me that answer makes it sound like the developers thought about adding Line Mode officially, but decided it was not worthy of that status. Line Mode does have flaws. You cannot see your ammo, crosshair, health, or any text while it's activated.