Is unopposed practice still useful at U12?

Short unopposed work can still be useful. The question is what you need it for and how quickly you bring the football problem back. If a player is learning a new movement, receiving position or passing detail, a few minutes without pressure can help them feel the action. Then I would add an opponent so they have to decide whether that action is still useful. I would be more cautious if half the session is spent repeating a pattern with no opposition and no decision. At U12, opposed football should do most of the heavy lifting because the game is full of pressure and choices. But “unopposed” is not automatically bad coaching. Use it briefly when removing pressure genuinely helps the player understand or feel something, then put the pressure back and see whether the learning survives.