I have been silent on this subject, but I just want to throw my opinion out there.
When I grew up, I got "discipline" a few times, but after that I knew how to act so it never was a consistent thing. And that lies in the line b/w discipline and abuse. People are still going to believe in the old school way of discipline, and I don't think there is anything wrong with that. The intentions are not to be an abuser, but to get the message across: bad actions have consequences. Having said that, I do think physically striking a kid should be an absolute last resort, if at all.
Other people are gonna throw the word abuse around too much. Abuse, is IMO, something so terrible and disgusting that it endangers a child's life, causes fear to the kid, leaves much more than some temporary scars, and it occurs on a regular basis.
I work in a school system, albeit just a sub, I've worked with many 4-6 year olds. You know what the best way is to discipline a kid is? Stand your ground. Don't give in, and IMO that's what many parents do nowadays. A kid throws a tantrum the parent throws them that piece of candy, buy them something, give their phone back, whatever. Tantrums are what kids do. Let them have their tantrum and they'll get over it. They aren't gonna hold a grudge.
I think another problem is how a parent is a seen in public when their kids act up. On one hand, the public wants the kid to behave, but on the other if the parents take some sort of action (even so much as gripping the kid's arm firmly and dragging them out) it can be interpreted as abuse. Then you got instant communication so that can be reported. It's a very dicey situation.