I dont see how this is needed. If a player is too far away then the boss should just lose any enmity and contribution that player has and then the boss focuses on the next person on the enmity table. Un escqpable mobs is just frustrating especially if you accidentally pass though one and can't run away from it. That can also be abused by griefers.

If a tank loses hate from a dps then they either need to learn to tank or SE need to fix enmity from tanks.