Specifically, this will happen with the first raid release.
In original Cataclysm, level 85 heirlooms were tied to guild perks. To obtain those heirlooms, players were required to join a guild and boost it to guild level 10, which was meant to be a significant barrier to obtaining them.
In Cataclysm Classic, we chose to make heirloom rewards much more available to players, and since our intention is not to have level 85 heirlooms available during the initial 80-85 leveling, we’ve capped heirlooms at level 80.
When the first raids open, heirlooms that are intended to max at level 85 will be increased to level 85.