Does IS help with slow moving subjects?

Here is the question the OP asked inside the post:

"But what about slow moving subjects in low light, for example school plays, people at a dinner party, award ceremonies?

Would there be a benefit from using a estabilized lens vs non IS?"

The answer to "would there be a benfie from using a stabilized lens vs non IS" is "YES" - in those specific circumstances, the OP would get a benefit - IF they can still get a high enough shutter speed.  If the OP is in one of those situations and can get say, 1/80 - 1/160 shutter speeds those shutter speeds will be enough to freeze some of the less animated action.  But, if they're using a longer focal length (which would be needed for school plays / award ceremonies) they might get camera shake.  IS prevents camera shake.

So - specific question is "Would there be a benefit from using a stabilized lens vs non IS" - the answer is YES.

Several of us have pointed this out.  And your reply is to quote the message title.  The message itself spells out the scenario the OP is interested in fairly clearly.  And, as I've said several times the benefit is in stopping camera shake -but it's still a benefit to the image which is what the photographer cares about.

Ask any wedding photographer if anti-shake hellps them for ceremony shots if they can't use a tripod or flash.  They'll say "absolutely".  Same thing here.