games-engines/openmw requires dev-games/openscenegraph-qt regardless of whether the qt5 USE flag is enabled or not. This dependency doesn't seem necessary because the game compiles and runs fine without it. It might not even be necessary even if you enable qt5, but I'm not sure of that.
You're right on both counts. In fact, it's only needed when qt5 *and* devtools is enabled, or at it would be if it weren't bundled in the extern/osgQt directory. Unbundling it involves more time than I have so I've just removed the dependency and left a note for now.
Maybe there is no sense in unbundling osgQt since it's unsupported by OSG team and is supposed to be bundled with applications. https://forum.openmw.org/viewtopic.php?p=53273#p53273 https://forum.openmw.org/viewtopic.php?p=56300#p56300 http://forum.openscenegraph.org/viewtopic.php?p=73094#73094
This should be revisited if upstream ever changes from osgQt to something else. For now, Michael is right, osgQt is removed in https://github.com/openscenegraph/osgQt/pull/22 (though dev-games/openscenegraph-qt doesn't include that change yet) and openmw has some custom changes to osgQt
