I have been lucky, not being punished for writing with my left hand. My mother was forced to write with her right hand. But even in the 1980s pupils in Germany were forced to write with the right (in both meanings) hand.
We now know a lot more, about what this does to a child's brain, but then, teachers just believed to be right.
I knew a left-handed math prof who had had that treatment growing up in Holland. But he'd developed a party trick.
One day he told us about this. And said that the result was that he could write with both hands. Different things. He demonstrated. Wrote with both hands at once, different things, on 2 neighboring boards. Kept on writing. Got to the bottom of the boards. Then turned around, began lecturing from the first board, reached the bottom, and continued the lecture from the second. He had laid out the boards perfectly to do this.
It doesn't do anything to a child's brain, it's just gratuitously cruel. That is reason enough. But many people the world over learn to eat or whatever with a certain hand and it does not hurt their brain.
We now know a lot more, about what this does to a child's brain, but then, teachers just believed to be right.