I have a Trek 4900 that was recently serviced for slipping. They replaced the rear hub and the chain. I am somewhat a novice at bicycle repair. The bike still slips and they cannot reproduce the problem. I have been trying to troubleshoot this and the only thing I can figure is the chain they put on a HG-58 chain when the chainring says "Use IG Chain Only". It is also possible the chain rings are worn out in which case: should I use this opportunity to change it out to a non-IG model? It is a Deore Mega-9 derailer in the back and a alivio in the front. Thanks in advance.

The one symptom that I remember from using HG chain on IG chainrings was that the chain would sometimes derail itself from the big ring down to the middle ring. The front derailer would then immediately try to shift the chain back up to the big ring.
But mine never slipped under very hard use in the usual sense, so perhaps the rings are worn.
I ended up doing a bit of file work on the right side of the big ring's teeth, to better retain the chain when it was crossed from the big ring toward the bigger sprockets on the rear wheel.

It would not be unreasonable to aske the shop who did all this work to take the bike on a brief test-ride, so as to properly diagnose this seemingly incomplete repair. Slippage could result from any of several different maladies, after all.

IG chains can still be ordered, and are fully compatible with 7 and 8-speed HG cassettes. I remember some riders preferred them for their slightly-narrower outside width and resultant more-forgiving shifting action. I often use IG chain on vintage bikes with great results.

Were "IG" chainrings anything special, apart from the marking? I thought they just marked them so that you wouldn't try to use a narrow chain with an IG cassette (fatter teeth).

IG was first developed for "compact" drivetrains, and the sprocket teeth were shaped to more-aggressively transport (shift) the chain at lower chain speeds.

The IG chainrings are marked so that you won't use a non-IG chain, exactly as it says.
The IG cassettes did have thicker cogs (more-pronounced shift ramp features on both sides of each cog) with thinner spacers, so the narrower IG chain wouldn't be so likely to rub against the adjacent larger sprocket, particularly during downshifts.

I don't know what the policy on necro-posting is here but just to follow up, because I am forgetful... I finally convinced the bike shop I needed an IG chain by printing off some documentation from Shimano. With the new IG chain on the bike has been working fine for about 3 years now.

Always glad to hear the rest of the story!