Does IS help with slow moving subjects?

Not a huge help generally.

The problem is that even people standing still really move quite a lot.  There's motion blur because of the slow shutter speed.

All IS does ( in any conditions ) is let you use a slower shutter speed but ONLY to avoid shake due to the movement of the lens and camera, not of the subject.

It's not completely without benefit because e.g. you might be able to get an exposure at ISO 3200 using a wide aperture in low light, but that gets you a shutter speed down at 1/60th ( just an example ).  That's too slow to freeze much motion but you can just about get away with it some of the time for people standing or sitting calmly,  As you might be shooting at say 100mm focal length you'd ordinarily need something like 1/100th - 1/150th of a second to avoid shake from lens and camera motion.  So IS lets you stretch that to the slow shutter speed you can get.

It's still a matter of luck whether you avoid the worst of motion blur for a small print, but you do avoid shake blur.

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StephenG