So I can tell you from TBC arena that mana drain channels cast before the druid shifted did indeed hit their mana bar in bear. Mana burn would fail. Mana drain while casted on an enemy in bear would fizzle. I’ve asked some old school pvpers whether or not sting went through, but none of us played hunter or druid.
Not sure how this behaved back in the day, but I know shadow form is doing the same thing.