Since the soon to occur addition of the "thirst" potion effect, the "hunger" potion effect added by Vanilla should no longer affect thirst.

This is not possible given the current way exhaustion works:

• The "hunger" potion effect does not actually decrement hunger, it decrements exhaustion. This is vanilla behavior.
• Hunger, saturation, and thirst all work the same way: they decrement when your exhaustion rolls over.

And looking through the call paths there is a very hard-coded behavior (see Potion#performEffect) where this exhaustion addition happens. I see no way in which we could nicely inject into this, nor do I think we should. The hunger potion effect does what it always does: add exhaustion. This in turns decrements your hunger, saturation, and thirst.