ThrownPotion cannot have a custom Item ID

In a previous Minecraft snapshot, 15w45a, ThrownPotion entities could be given a custom Item ID which would override the splash potion model when thrown.

e.g. /summon ThrownPotion ~ ~5 ~ {Potion:{id:"minecraft:diamond_sword"}}

This was probably to fix the bug where thrown lingering potions would display the splash potion model, however, it opened up a lot of cool possibilities to use this feature creatively and was generally considered a great addition, making the possibility of custom thrown models a reality, even if they did have a rotation lock.

In the recent snapshots, from 15w46a onwards, there's no ability to use a custom id, even though the lingering potion displays correctly and uses the lingering_potion model.

Was there a reason this feature was removed?