so, i just decompiled pl_promotion (just for knowing how to do it) , and checked out the ice area
there were a few things there: a func_smokevolume covering the ice area (for making it look misty i suppose), and a custom texture called agent_ice2
pakrat gave me agent_ice2.vmt, and the only odd things there are $dudvmap (which the valve wiki says is for refracting on dx8, so no ice) and "$surfaceprop" "ice" (which isn't strange, but it's the only thing i can find that might explain ice). valve wiki says $surfaceprop can be used for friction, so $surfaceprop ice should give you ice if you want it!
however, just because you know how to do it doesn't mean you should. as others said, ice probably kills player fps and/or network connections (depending on whether it's calculated by the player or the server), and it would be hard to maneuver on it.