When did this become a contentious topic.

When people decided to make the perfect the enemy of the good. I don't know what concise term it is that is happening here, but it is the same as Time Out. So before Time Out was a thing, parents would assault their children simply because they, the parents, felt upset or frustrated with their children existing as new people who were still learning. In order to keep children from being hit so hard they'd welt or bruise, the public health authorities convinced parents that Time Out was by far more effective -- go sit in a corner by yourself! The only thing it was more effective at was getting that child out of the walloping range of an emotional parent; the goal. But if the public health authorities told those parents that, they'd keep hitting their kids. Time Out is not actually an effective teaching tool and for parents who aren't inclined to beat children, it's actually a poor choice to make vs taking the teachable moment to teach. So it isn't really A Good Thing as it has been branded. But if it saves even one child from harm, let's spread the good word.

Same with this one. Some children were overlaid by inebriated parents. Saying that children in the bed is A Bad Thing will save the lives of children whose parents are prone to inebriation and would otherwise have bedshared with them. And since it saves even one child from harm, we spread the good word. However, much like the parent who isn't going to beat their child above, there is also the parent who avoids intoxicants.

Yet people like hard and fast rules and like sanctimony. So they'll stan No Kids in the Bed under all circumstances without noting the nuance.

Personally, we follow the Safe Sleep 7, though baby spends more sleeping hours in his own bassinet than in our bed. Yet sometimes, only proximity to parents will do at night and so we make it as safe as possible so we can all get some much needed sleep.