I was born in ‘74 in Southern Cal. Spanking was universal amongst my friends growing up, and there were constant references to it by those in authority. It was also understood that any adult could spank you if they saw fit. This was made clear to me a few times when I complained of babysitters, or friends parents threatening to spank me. (The threats worked, but I thought it was unfair for them to spank me...my parents did not).
As others have stated, AC was not common back then, so most people had their windows open in the summer months especially, and hearing spankings coming from houses was common. It was a source of great embarrassment to me with my own spankings, as I was very vocal and almost sure I was heard every time I got spanked. I can remember being spanked and going out to play later and having neighbor kids snicker at me, mock me by patting their butts and howling, etc. Of course, they got it sometimes too but that didn’t stop kids from making fun of you.
I also witnessed a few spankings, but not many, and that I can remember right now, no one outside of my family saw me get it.
Public schools for the most part did not use spanking as discipline in CA at that time, although a couple of my friends did get paddled in sixth grade, it was very rare. However, birthday spankings were given by teachers in grade school, the birthday boy or girl would be called to the front of the class, placed over the teachers knee, and given the appropriate number of spanks on their clothed bottoms. In second grade, you could choose a “spanking tunnel”, a game where the kids would form two lines facing each other, join hands, and as the birthday boy or girl passed through everyone would try to hit their butt. The girls mostly chose to let teacher spank them, but the boys were egged on by the other boys to pick the tunnel. I got a couple of less swats taking the tunnel than going over the knee.
Speaking of which, EVERY birthday party I went to included a spanking for the birthday boy. Usually one person would hold him down, while everyone smacked their bottom. Parents would usually not participate...because they would do it another time, presumably BARE. I can remember one of my friends in particular being told by his mom “I’ll get you later” and the boy smiling and blushing bright red. My own parents were not big on birthday spankings, but allowed them at my parties, and my cousins always made sure I got them.
So how common was spanking in the 70s/80s? Pretty much universal, I would say, at least where I grew up. In sixth grade my teacher asked for a show of hands on who got spanked at home. Almost every hand went up. Mine did not...but I was lying! (Trying to be cool). In 10th grade, my sociology book said 50% of high school seniors reported being spanked at home.
You can also look back on Diffrent Strokes and Good Times episodes that had many references to and jokes about spanking. I remember watching Arnold get threatened with spankings all the time.