Not sure whether I take an absolute stance on smacking or not. I grew up in Sweden where even in the 60's it was very uncommon. By the time my nephews or nieces were born in the 80's it had been banned. So it was never something I thought I might come to depend on; there were so many other things that people did about discipline and had been doing for a long time. We never knew the ban was meant to mean that children had to be allowed to run riot. IME Swedish children are not worse behaved than English ones, and you do see a lot of very good, firm parent behavior, also a lot of good parent-child interaction. So smacking has not been part of my parenting methods.
On the other hand, some of the parents I know in this country are what I would call excellent parents, even though they have been known to smack. So maybe it's not an absolute.
Have to say I would feel dubious about someone who sees smacking as a normal part of a discipline once the child gets beyond the age of 7/8, as this seems to suggest either a lack of understanding of child development or sheer desperation. By this age, children should have developed a sense of dignity which makes physical punishment humiliating; it's not just a physical thing any more. IME smacking a child of this age or over is also quite rare, so the child will feel different from her peers.
I am a little surprised that the thread about the spitting 11yo should have gone off into a general discussion of modern discipline: what most people seemed to miss was that this was a situation where both mother and child were under extreme stress due to a serious medical problem in the family, and the child was acting hysterically in a way she would not normally have done. This wasn't about general parenting or child-parent respect. They both needed sympathy and help, not least the mother who was struggling with a very difficult situation indeed.
If you look at the "But we took you to stately homes" thread, I think there are absolutes to be found there. Humiliating somebody, belittling them, making them feel they can never be good enough. Sexual abuse is another absolute, I would say.
I suspect the reason smacking has come to dominate the thread is that this is an area still open to debate. No sensible person would disagree about the evils of sexual abuse or constant belittling (though with the latter, it's surprising how otherwise sensible people can slip into the habit without seeming to notice).