A lot of it is going to depend upon mobility.
1% of the population is still going to leave you with 70, 000, 000 human beings. That’s more than the population of the entire Earth during the dark ages.
Until the late 1960s, Australia was an entirely self-sufficient nation with world standard technology and a population of about 7 million. So if you can get even 10% of those people together in one place you can quite easily maintain 1960s level society and technology. Given the various advances that have been achieved since then, I imagine that maintaining 1980s technology wouldn’t even be difficult.
It’s impossible to say whether this would happen or even be possible.
Initially the world would be fairly chaotic. However people will concentrate on any areas that has food and medical services. If it was just a plague that caused the population collapse then the use of radios and aircraft would rapidly bring together survivors acrosslarge regions. A group of survivors would broadcast to others, and aircraft would rapidly allow any people without radios to be contacted. Within a couple of years we could reasonably expect that 90% of survivors in the developed world will be living in the same region and functioning as a society.
So In North America for example, we would expect that most of the 5 million or so survivors would all be settled in some productive region, likely Florida and surrounds. Assuming the survivors were random individuals there would be sufficient doctors, engineers and so forth to easily maintain post-war technology levels. Raw materials would be no problem at all for the first few generations because everything needed is simply lying around for the taking.
The biggest unknown would be how people actually react. 20th century technology requires stable government, functioning economic systems and so forth. Those things don’t just spring out of the ground fully formed, and with a total societal collapse it is not a given that people will immediately adopt the societal conditions needed to maintain the technology base.
My personal belief is that people would flock together, then some group would attempt to become the new leadership. This is very likely to be the military. We would then see a military dictatorship arise, with a sizable portion of people choosing to flee into the wilderness in response, but the vast majority staying because of the better services offered by the larger community. While such a dictatorship could maintain technology levels for the first generation, I am skeptical of the ability to implement the economic and educational systems need to prevent regression
So 20 years after the event I would predict the vast majority of Americans, Canadians and Mexicans living in Florida. There would be a military dictatorship with working 20th century technology. Generally things would be very similar to what was experienced under European Communism or wartime Fascism, with a moderate standard of living but few luxuries, considerable rationing and obligatory work details. There would be pockets of dissenters living outside the controlled areas, but likely nearby enough to maintain trade. Most of these groups would probably be tolerated, though treated as foreign nationals, provided they didn’t provoke the government of the day.