It's interesting how social norms changed our perceptions. What was 'normal' even a generation or two ago (more or less depending on one's specific culture) is now considered abusive. Go farther back and it's more intense. Ever read the "Little house" books? Very wholesome. About very loving families. Children's books, and ones I would honestly recommend for kids, wholeheartedly. And yet, the books talk about hitting children with switches and whips.
What I'll say is, from what I've seen (and, what I've experienced), yes, there's lines, and yes, there's gray areas. However, what seems to be the biggest dividing line between whether or not it's actually experienced as traumatic, is how it's implemented, and what the overall family culture is. A loving and supportive family culture with clear expectations of rewards and punishments is much less likely to be viewed as traumatic as one which is cold, angry, chaotic, etc. and where punishment is handed out according to the moods of the punisher, not according to the behavior of the punishee.
Because of this, you sometimes get the interesting cases of people defending having had harsh physical punishments in what may well have been a healthy family environment, yet you get people who were given comparatively mild (or no) physical punishments in a very unhealthy family environment who were traumatized by the ways they were disciplined. Which is of course not to advocate dangerous beatings or extreme physical punishments- or even to advocate spankings (I'm not sure how I feel about that practice, anymore, tbh, though I am less opposed to it now that I understand the bigger-context issue). But, it's kind of interesting, I think.
Just my own somewhat-anecdotal $.02.