I don't know the best answer to this. I don't want to blanket say "it's child abuse" but it feels like it doesn't work.
As a kid, looking back on it now, I was pretty terribly behaved and my mother would give me a smack once in a while but my father never ever did anything like that. They didn't beat me up or anything. We are talking about 1 hit when I really stepped out of line.
I don't recall that it ever stopped me though. I was misbehaving right up to my teens but then the misbehavior really escalated in my teens and I really never got a grip on my life and my behavior until I started working, really.
Sure I turned out OK now but I wouldn't say it was related to getting a clout from my mother once in a while. If she had beat me up for real would it have made any difference? I doubt it. Probably would have made me worse.
However, I have some serious resentment and long lasting anger built up over punishments that were not physical and, in my opinion, have had a profoundly negative impact on my life. They employed punishments that were sometimes highly humiliating and embarrassing and I think those fed into some seriously bad behavior.
People also need to consider psychological abuse here too. There are parents who won't hit their kids but for sure will do things that are still strange and unusual punishment that simply gets a pass because it isn't physical punishment.
As an example, if a parent completely humiliates their misbehaving child in public could this actually do more long-term damage than a smack?