I agree that this is an extremely annoying change, the one major disappointment in the game for me.
I've returned to the full release after a long break and find the group assignment system over-complex and under-featured (and frankly broken, unless the features I want are hidden under the complexity). I'm doing a "train the village recruits" mission; previously I'd have made a group called Recruits that could be handled appropriately to the mission goal, now they're mixed among the rest of the low-tier infantry with no obvious way to keep them safe.
I suppose all we can hope for is a mod to fix this, but then from what Agroplode wrote above, if the AI relies on the implemented system (i.e. can't cope with player-assigned formations) then a mod might make things worse.