Wrong wording. Obviously it WAS done but you didn't heard about it nearly as much nor was it widely publicized how,what,where,when,ect... Also you had copper to treat with and not much else. My point was it has come along way.
The need for a proper quarantine was as well publicized in books as it is today, but back then you didn't have the internet to share your experiences easily on forums and message boards.
Copper was by no means the only measure of treatment, though I agree that back then it was seen as the cure-it-all go to treatment. Unfortunately this is still the case with many today.
Formalin, the different dyes (malachite green, methylene blue,...), Acriflavine, and the like where already well in use, though mostly by more experienced aquarists or those organized in clubs (and in Germany there were many of those)
Also Chloroquine Phosphate was already available, even if not widespread used for fish.
The things that really have come a long way are antibiotics - that was back in my beginner days rather trial and error and for sure nothing for beginners.
The real difference today is the widespread use of the internet that allows easy and quick sharing of experiences.
I asked this about expensive fish they sell since most of the time you call and say the fish didn't make it and they offer a refund or to reship the fish in question with no body needed. Just goes back to my question of if its worth spend 4K on a Clarion from DD for the 2 weeks or to buy one that costs 1700$ and doesn't have the two weeks... I think everyone gets that
Two weeks is not really cutting it with most fish, especially with super expensive ones. It's basically only protecting you against the fish coming with something like a deadly brook or velvet infection.
A fish caught with poison, one that never starts eating or one with a more slowly progressing disease may easily hold out longer before slowly wasting away weeks or months after the guarantee run out.
A guarantee would need to be 2 months (the minimum quarantine period IMO) to be of any value. Though no one will give you that since there are too many ways the "end-user" may screw up. The latter is the main reason why most stores only give you a guarantee on their livestock to their front door.
So I would rather go with the cheaper fish, at least as long as I can pick it out in person. And if I had the money to shell out on such a fish I would add the few bucks for a flight ticket to go there and check it out myself.