I'm undecided on the question of whether it's ever justifiable to physically discipline a child, but I certainly think there are nuances based on personal experience.
I was given a "tanned arse" by my DF on, from what I can recall, only two occasions, both times with a slipper. I can't remember what it was for, but it will inevitably have been for the usual backchat, being told a million times and not paying attention, or incessant squabbling with my sibling. In any case, it's not something I ever spend any time thinking about unless the topic is brought up and it's prompted. It's a total non-factor in my life.
At one point in my teens DM took to frequently, and repeatedly, smacking me around the side of the head until such point my ear was bright red, ringing, and twice it's normal size. This was for any sort of perceived transgression, not necessarily something that was actually true and often for things that she'd decided I'd done when I actually hadn't. This was always accompanied by a finger wagging, peering in until her face was no more than a few inches from mine, and a screaming, ranting lecture about my failings as a child and inability to just be normal. Things I can remember being smacked around the head for include leaving a teaspoon in the sink, using the telephone when I actually had not, skipping school one day (fair enough, caught me), and no doubt a load of other perceived irritants and things that DM disapproved of.
In total contrast to being slippered as as child, I absolute do remember my DM's behaviour when I was a much older teen, I absolutely do resent her for it, I absolutely do think it was unreasonable and did so at the time, and it's also notable that it stopped as soon as I was significantly taller than her and made noises about not being willing to simply stand there and take it. She has never even acknowledged this behaviour, never mind explained it or apologised for it.
The first I consider perfectly reasonable discipline that hasn't harmed me in any way, the latter, an assault on a child that has caused long-lasting resentment and contributed to a completely broken relationship.
So yes, I think it's important to differentiate between "smacking" a child for disciplinary reasons, and "assault" or "beating" a child, and I do get annoyed when any and all physical discipline of a child is described as the latter.