Not a physicist, but I thought the heat death of the universe also involved all the matter being sucked into black holes and turned into pure energy. There’s a big chunk converted up front in the accretion disk, then the rest is converted into hawking radiation as the black hole(s) evaporate over the oodles and oodles of years.

Whether or not there are also lumps of iron-56 or other matter floating around in the cold void probably depends on the real truth behind dark matter and dark energy and their long-term behavior.