Despite the fact that these countries generally had age of consent laws, it seems like the producers of such material operated in the open with few or no legal consequences. In addition to its infamous bestiality films, the Danish Color Climax Corporation released films of girls as young as 7 being molested, and, while the production of such films ceased in 1979, the company still exists to this day and I've never heard of those responsible for making the videos being charged. Dutchman Joop Wilhelmus became a millionaire from pornography, including the child abuse magazine Lolita, which ran for 17 years from 1970 until it was finally shut down by police in 1987; Wilhelmus was arrested in 1971, but released, and even appeared on radio in 1975 talking openly about molesting children. He was later imprisoned for abusing his 12 year old daughter, and drowned while drunk shortly after release. In the USA, I have seen articles claiming that 1970s Times Square sex shops were openly selling "kiddie porn" for up to $50, including a film called Preschool Sex, which is exactly what you'd think it would be.
This seems absolutely shocking to us today, but at the time such practices must not have had the same social taboo.
Additionally, I'd also be interested to know what the legal situation around such material was in Japan before 1999, when the distribution of child pornography was explicitly criminalised. Prior to that, "obscenity" laws existed which were sometimes used to prosecute producers of such material, like a man in 1993 who produced a porno with an underage performer, but was there ever widespread legal or semi-legal distribution of serious child abuse material in the way there was in Denmark, the Netherlands, and the USA? The 1970s American articles I have read make little to no mention of Japanese material, primarily referring to that from the USA and Europe.
I am aware of the Color Climax productions. I believe they were and remain a legitimate studio that, as a company, ceased those productions that were made illegal. I am not familiar with what, if anything, may have happened to the persons directly involved. As far as Japan goes, I am not aware of any similar commercial activity, however there is something called "idol" (basically any kind of modelling, I'm not sure if it's a translation thing) where some of the magazines etc. where claimed to be questionable. I am not aware of any legal action that was taken. In any case, these materials are now included in databases for image matching algorithms in detecting and prosecuting cases.
In terms of social taboo, efforts to enact laws were initially focused on preventing sexual abuse, not on finding and prosecuting sexual deviancy or assumed deviancy. So, investigations into Color Climax and similar, while slow to start, did proceed and close. It wasn't for another 20 years or so that possession of materials was outlawed and the contemporary fear over underage (however that is defined by jurisdiction, another complication) sexuality began.
It should be noted that commercial child pornography outside of Color Climax and a few other notable cases has never really been a thing. CC was in fact a big part of the political push to ban commercial child pornography. Except for a few enterprises out of eastern Europe in the 90's/early 2000's, there hasn't been anything notable of the sort since CC. Most imagery deemed illegal today is actually self produced or incidentally produced (webcams, phones etc.) and is proliferated on the internet without economic incentive. When money is involved, and likely abuse as well, the most common context is prostitution. The reason we have many prosecutions and hear about this so much today is both because possession was not made illegal until the 1990's, many years after the sources of commercial material were shut down, and because the internet has made access trivial, free, and widespread.