I finally got a hold of my old PS2 copy of the Shadow of the Colossus demo. Is it true that there is an old developer menu you can access? If so, how do I do that?
Shadow of the Colossus had two demo versions. I believe one was an early trade-show floor build, and hard to come by. The other, which is what you probably have, shipping on a demo disc from the Official Playstation Magazine (or some other publication of that era).
I don't believe either version had a developer menu (I've never heard of one). Most of the time, such menus were accessible via GameShark to Action Replay codes. But I don't recall every having come across a code to enable such a menu or any other sort of broad "developer" functionality.
Indeed, if you peruse the game's entry over at The Cutting Room Floor, you'll see that there's a fair bit of cut content, but no serious mention of a developer mode in either the main game or any of its known demo builds.