But if the base game engine they create doesnt support climbing (devs dont put it in), then the people who create the game world cannot utilize whats not supported. So if the game engine does not support climbing, we will not see climbing.
So it is on what the engine can and cannot do, but its also up to the devs to decide what those things are if im not mistaken.
Gamebryo can't do anything related to gameplay, it can only draw polygons. It can't handle sword fights, it can't handle walking, running, jumping, swimming, it can't handle conversations and so on and on. If you do something in Oblivion, then the graphic engine can't do it, cause it's not what a graphic engine do.
In any case, one of the very first games that used Netimmerse (which is what Gamebryo used to be called a decade ago, Gamebryo 1.0 is basically Netimmerse 5.0) is http://en.wikipedia.org/wiki/Prince_of_Persia_3D, and it http://www.youtube.com/watch?v=XCJKOMt8aAc.