The physics engine I use in Leviathan. Doesn’t have an “up” direction, so by setting the gravity vector to what ever you want, you define that as “down”. So there isn’t any reason to not do the coordinates the Ogre default way (also SFML sound and cAudio define +y as up).
Pretty much the only libraries that don’t by default have +y as up (most libraries let you set the up vector to be anything you want) are ones that want to use the DirectX convention.