I had cause to call Halo tech dept. a couple of years ago to determine if it was okay to install IC cans in a 2x6 ceiling/floor system. In this installation the top of the cans were literally placed directly up against the plywood. I, too, could not find any mention in the instructions for clearances except specifically for insulation. Ultimately, after asking around a bit, I had sufficient uncertainty, so contacted the tech department. I was told by the tech that the cans may come into contact with any combustible building material normally found in construction.
Still, the ABS question in the OP is a good question because ABS has a low enough ignition point as to be banned altogether in many places.