A couple of things. First, my wife and I do not spank our kids or use physical punishment of any kind. Most mechanchim I’ve spoken with are very much against it for today’s dor.
My father did spank me, though not too frequently (maybe a handful). IMO, it hurt my relationship with my father for a long time, though I only realized that as an adult. I know he meant well and it wasn’t anything near what I would consider abusive, but I think it distanced us from each other a lot. Having said that, I think it does depend very much on how it’s done (the manner in which it’s done, until what age, etc). I was threatened with it as late as about 12, but I think he last did it when I was 10ish, which is probably too old no matter what.
So to sum up, I think it’s possible to spank your child in a productive way, but it’s very hard, and IMO not worth the chance of messing things up these days. Also, I think anyone besides a parent doing it is off the table completely. No way a non-parent these days can build up a healthy relationship with a kid and show love in such a way that makes physical punishment a clear act of tough love.