So I'm ending this thread here. The AAP has a lengthy position statement on discipline and how to effectively do it. Read up. Tons of well structured studies have led to their positions and recommendations, and lend much more weight than our anecdotal case reports: https://pediatrics.aappublications.org/content/101/4/723
I will say I was a fucking terrible kid. I got so many spankings I can't even count and they fucking hurt. Soap in mouth? Yep, bar and liquid. Timeout? Who didn't. Not sure if the spankings even did anything for my behavior because I faked it half the time to have them stop sooner. Also there's plenty of research to show that this isn't the best approach for dogs either yet people still do it.
There's not a fine line in discipline as some here have said. It's pretty fucking obvious. You ever see a family in public berating their 3 yr old for something they can't even understand yet? Yeah too far. Ever see physical punishment in public? If it looks like it went too far, it probably did. Corporal punishment is controversial but it's pretty clear when you cross the line from punishment to abuse and it will have all sorts of negative impacts on your child's development and performance. I've seen all kinds of parenting strategies and it's astounding at times how fucking stupid the parents can be. Also seen way too many times what happens when parents (or boyfriends) go too far.
It sounds like there's a mix of opinions in here (old school, mixed, progressive, and Russian), and everyone is free to do what they wish. If you haven't had kids yet, when you do, I highly suggest reading that link above for advice on how to discipline your child to have best outcomes. Take it or leave it.