It appears based on the description in the character creator that the Mage Slayer penalty has been changed from a -50% beneficial effect duration to the same chance of spell failure occuring on beneficial spells cast on the Mage Slayer as hostile spells. However, in game a global -50% beneficial effect duration is still applied.

I would like to request in addition that Frenzy not be reduced in duration because of the Mage Slayer reduction of buff duration. It seems sort of ... wrong to me, plus it makes the entire class less appealing when the main thing that makes Barbarians... Barbarians is half as effective?

If this is a bug, that will be changed too. Because it's only half duration because of the 50% beneficial effect duration reduction.