Depends if you think babies and moms dying is a morally bad thing or not.

Estimated 30-50% mortality rate for babies and 1% per birth mortality rate for moms (times say 12 kids). Lotta innocent babies gotta die to achieve your goals.

While this may sound tongue in cheek or a gotcha. I do think people underestimate how terrible, hard and brutal it was to live in those cultures as a human. We definitely create our share of travesties as modern humans (factory farms, anthropogenic mass extinction events, global climate change making the world uninhabitable). It remains to be seen if we can use technology to out pace some of these problems we've created, but I'm somewhat hopeful that we can, and find a path forward to live humanely and sustainably.

People always think about medical conditions when thinking about archaic societies; but there is also an associated cost in not having a central ministry in charge of monitoring violence to women. Energy abundance shaped people’s moral values and society, we’ll also lose them when scaling down. In other example: Who cares about PETA during war, who protects the gays, etc.

So yes, scaling down is de facto against our moral values.

A pure hunter gatherer lifestyle is going to support a percent or less of current population. So you are basically going to have to prohibit the vast majority of the population from having children for a century or so. It also means abandoning all technology. Good luck persuading people to go along with that.

Humans produce the vast majority of food we eat.

If we stopped that, and just consumed the food that nature spontaneously produces, it supports maybe 50M people. So first you'd have to kill 7000M people.

Even after that, I don't know how you'd stop people from reinventing agriculture.

So on balance, I recommend against this :)

How many billion people would need to die to make that sustainable?

This sounds like hyperbole but it’s the honest truth. Billions would die due to food shortages. Our modern farming system is incredible at its job of feeding 7 billion people.

...and yet 1/3rd of the food is thrown away/perish globally.

There is a lot of excessive waste / inefficiencies due to energy being -- relatively -- cheap.

If energy prices soar, many inefficiencies will get more attention. Like, if food becomes more expensive we would stop throwing away 1/3 of it. (And more houses would get proper insulation. And so on.)

At least part of which is a necessary hedge against possible years with a bad harvest, though. A temporary shortage of toilet paper or even microchips is one thing, but you most certainly wouldn't want the same thing to happen with your food supply.

You want to reduce the world population to a hundred million or so? How do you do that ethically?

How would you get everybody to agree to accept all the limitations and ills of their non technological lives without wanting to reinvent modern civilization?