This is a subject that is near and dear to my heart and I'm so glad you asked. I am a foster and do not work at a shelter so I do not have any firsthand knowledge about changes in behavior problems but I have been studying early socialization for over a decade. I believe it is the key to preventing behavior problems and, ultimately, to reducing the number of adoptable pets that are euthanized each year.
The American Veterinary Society of Animal Behavior states that behavior problems are the #1 cause of damage to the human-animal bond, surrenders to shelters and euthansias. I don't think shelters like to highlight this fact because it gives the impression that shelter dogs are 'broken' which, to some degree, can be true. However, with an appropriate dog-family match and a little guidance and training, most shelter dogs still make wonderful pets. But why not prevent these problems in the first place by socializing them? I don't think we can stress this fact enough: Behavior problems are the #1 cause of death in dogs under 3 years old in the US, not infectious disease.
I attended a seminar by Dr. Ian Dunbar in 2009 where I learned about early socialization (0-12 weeks old) for the first time. This was after a lifetime of interest in animals which included volunteering for animal shelters, participating in 4H, attending horseback riding camp, owning a dog walking business and earning a BS in zoology. Almost no one talks about early socialization. Puppy Culture and Avidog are the only two resources that I have found and these programs are both geared towards breeders. The idea that, with a little extra effort in the beginning, we can provide lifelong, positive impacts AND reduce the number of animals surrendered and euthanized has changed my outlook on animal sheltering and fostering.
I think the source of insufficient puppy socialization stems from a combination of ignorance and fear. (I just want to note that ignorance is not the same as stupidity. We are all ignorant of 99.9% of what is knowable.) Most puppy owners, breeders and animal rescues have almost no understanding of early socialization. I knew nothing about it and I have a degree in zoology! We are also afraid of puppies getting sick and, fair enough, the death of a puppy is heartbreaking. But do you know what is also heartbreaking? A pet being surrendered to a shelter. It devastates the pet, the family, the shelter staff who have to look that sad, scared, confused pet in the face and the person whose job it is to euthanize them due to overpopulation.
Socializing a puppy before 16 weeks old is an act of courage - we are bearing the burden of the possibility that a puppy will die to prevent the near certainty that they will develop behavior problems. I think, as a society, we have been lulled into the misconception that if we just play it safe, nothing bad will happen. The problem is that 'safe' is an illusion. There is a cost to every action we take (or don't take) and we all have to play the never-ending balancing act of costs vs benefits.
When covid first hit and we started to lock down and socially isolate, I became concerned about the effect this would have on puppies and young children. I have no data on dogs but we are discovering that children ages 0-5 are experiencing an average of a 20 point drop in IQ as a result of isolation and masking (not being able to see expressions or peoples' mouths when they speak). Since dogs are social creatures, I would assume they are affected similarly but it is quite unpopular to point out these inevitable consequences.
Dr. Dunbar recommends that puppies meet at least 100 people before 8 weeks and another 100 before 12 weeks. To do this we have to either invite people to our homes or take puppies into public. Yes, by taking puppies into public before they have their final vaccination at 16 weeks we are putting them at some risk. BUT, with some very basic precautions (don't let the puppy touch the ground in areas that are frequented by other dogs, for example), that risk pales in comparison to the risk of a puppy losing its home and, potentially, its life.
Dr. Dunbar's seminar actually changed the direction of my career and caused me to start World Ready Pets. My goal is to create a video training series that makes it easier for more people (fosters, breeders and pet owners) to raise a well socialized litter of puppies by taking advantage of the first half of the early socialization period (0 - 8 weeks). There is very little information available on how to raise puppies because, well, it's not easy. They require a lot of work and space but, with this video training series I will try to break it down so that the puppies get what they need in terms of behavior and socialization. I also hope to encourage more people to foster puppies.
Just thought I'd share some thoughts since it seems that you see value in early socialization and are concerned about the effects of isolating puppies. It's a topic worth discussing.