As of February, 2014 this has changed:
https://forums.station.sony.com/eq/index.php?threads/changes-with-the-february-update-developer-updates.207587/
and more specifically:
"Finally, with the reduction in procs to the Headshot, Assassinate, and Decapitate abilities, we felt comfortable making these abilities more relevant to current EverQuest. As such, these abilities at their maximum AA will now work on mobs up to a player’s current level -1. For example, a level 100 Rogue with maxed out Assassinate will be able to assassinate level 99 mobs."