Considering I never noticed it until now, it was probably better to leave it in rather than screw up the composition of Decker walking away down the corridor on the right (and the arm goes away within a few frames.)
I noticed when watching "Alien" on DVD years ago that at the end of a handheld POV shot of Ripley running down a hallway, the set basically ends in blackness but through the steam and smoke, you can see an arm waving a black flag in front of a light on the extreme left edge of frame. Not sure if that was digitally erased when they did their major remastering a few years ago but I seem to think it was.
On scope, just like the other sound formats, the o-neg has a bit of info on the left edge that is normally covered by the soundtrack, but there is no reason to be accidentally showing that on TV. But there isn't really anything outside of the right edge of theatrical.