There is clearly a backstory here.
Acceptable behaviour does evolve through generations. There are many things our parents or grandparents did that we would not or could not - both good and bad things. Previous generations played out more than current children, for example. Previous generations smoked indoors surrounded by children. Even 15 years ago when my eldest was a baby, you could smoke with a baby in the car - something now allowed now.
So it is right that things used to be considered normal and accepted, that now seem horrifying. Child physical punishment is a contentious one of those.
I raised in the 80s. I would say that being slapped by your Mum wasn't seen as anything horrific then. But there was a line between a slap (or whatever) and child abuse. I guess the police's job was harder back then because of the bluring of that line between acceptable punishment and abuse. But abuse existed and was unacceptable in the 80s, it wasn't tolerated like a slap might have been.
So I guess much matters in what your Mum and you considers a "thrashing". If it was abusive, then no everyone was not doing that. Equally while there would be some families who never physically assaulted (as it would be called now) their child at all. I wouldn't think this was a majority though. I was bought up in a naice middle class family with two loving, involved and engaging parents. I think I may have been slapped 4 or 5 times during my entire childhood. This seems very slight compared to friends at school. My husband (also raised in the 80s) was beaten much more, but again within the relmes of normal for the generation, and feels the same as I do.
I have four children (ranging from 6-16) and have never physically hurt them. That doesn't mean I look down on my Mum or mother-in-law because they did - it was of a different time. I wouldn't expect my Mum to need to apologise for the times she slap me. Because she didn't know she was doing wrong, at the time the type of child punishment she did was socially accepted and normal. I think anyway.
I'm sorry you're feeling distressed by your past.