What country are you in? That’s definitely not common in the US.
Running across a doctor telling you to do anything other than back only sleeping is like running across a doctor telling you that vaccines cause autism. I’m sure it happens, but it’s very rare.
I’m in the Netherlands. I also was not referring to doctors. We have help, kind of nurses, that comes help out for about a week after birth to get the family back on their feet and up and running again while they take care of the household (secondary) and the health of baby and mother (primary). Although they obviously get medical training, they are not doctors by far.
I think your original anecdote that the advice seems to cycle is likely very specific to your situation. My wife is a pediatrician and she's certain that in the US at least, the recommendations both official and actual haven't cycled for decades.
Running across a doctor telling you to do anything other than back only sleeping is like running across a doctor telling you that vaccines cause autism. I’m sure it happens, but it’s very rare.