I am remembering my friend's son who would punch her in his autistic rage when younger. This didn't stop until he was around 17 years old. Sometimes she was black and blue and definitely traumatised. I don't believe she ever hit him and I don't think it would have helped but I do think it's a good example of a parent needing to defend themselves against their child.
My son has just turned 13 years old and is already taller than me and has a longer reach. I am grateful that his autistic rage has always been limited to shouting and pacing (although that can be pretty wearing) as I don't know how I could defend myself if he became physically violent.
We have never used smacking as part of our discipline with our DS but then he talked early, his autism causes him to be quite rule-abiding and we have generally been able to reason with him. Smacking? Just no real need, fortunately. With a different child, it might have been a different story. Who knows?
I sympathise, hugely, with people who have described lasting damage caused to them by parents who abused them regularly in anger. I feel there is no excuse for this type of abuse.
However, I can also relate to people who experienced smacking as a rare disciplinary measure that they feel caused them no harm.
I also think that many of today's children (my DS, for sure) would envy them their opportunity to learn effectively in well managed classrooms but how to have good discipline without the possibility of abuse?