Societies and cultures compete with each other. If a norm is truly beneficial for society/culture, then societies/cultures which maintain it will outcompete in the long-run societies/cultures which abandon it. The more successful societies/cultures will spread the norms which enabled their success, through emigration, war, diplomacy, trade, investment, emulation, etc. Whereas, if a norm is harmful or irrelevant, that won't be true. Natural selection at the sociocultural rather than biological level.
Burning witches doesn't seem to give any society a competitive benefit – certainly not in contemporary circumstances – so it is unsurprising it has mostly died out. While many ancient, mediaeval and early modern empires built their wealth on the backs of slaves, in late modernity slavery appears to be more of an economic detriment than economic benefit – a free workforce has greater flexibility to respond to changing market demand for skills than an enslaved one does – so it is unsurprising it has greatly declined–although, contrary to what many think, it still exists, and still even has its defenders.
In terms of what is "good"–if you believe that there is some deep metaphysical connection between the True and the Good, such that even though they are non-identical in the past and the present, in the long-run they must converge–the sort of view presupposed by that Martin Luther King Jr quote which Obama liked so much, "the arc of the moral universe is long, but it bends toward justice"–then if a social norm helps a society/culture outcompete in the long long-run, it must be good.
Whereas, if your metaphysics has no room for any such connection, then what social norms turn out to be most beneficial in the long long-run, and what social norms are actually good, may be fundamentally unrelated, even in complete contradiction to each other. Maybe witch-burning and slavery are simultaneously great evils, but also will inevitably conquer the earth because of their great advantage to any society that adopts them? I hope not. A worldview which would permit such a conclusion is rather dismal.
Martin Luther King was a Christian, who believed in a moral order imposed externally by a Supreme being. If you purport to not believe in some sort of religion (loosely defined), how can your metaphysics allow for social norms to be judged by anything other than some sort of empirical fitness function?
> Martin Luther King was a Christian, who believed in a moral order imposed externally by a Supreme being.
A person doesn't have to be a theist to accept King's principle. A naturalist/materialist/physicalist metaphysics doesn't have room for such a principle. But theism and naturalism/materialism/physicalism do not exhaust the space of metaphysical options.
A person might believe in the Scholastic doctrine of the convertibility of the transcendentals – that, at an ultimate level, Goodness, Beauty and Truth are identical to each other, even though at the non-ultimate level they are distinct. It is easy to see how a person who accepts such a doctrine might be attracted to King's principle as an expression of it. While the Scholastics themselves accepted that doctrine in a theistic context, and even saw theism as a necessary consequence of it (ultimately, Goodness and Beauty and Truth are identical to each other, because they are all identical to God), a person might accept that metaphysical doctrine yet find some rational reason to refuse its theistic implications.
Also, I can't speak for King personally, but the idea that morality is "imposed" by God – in the sense that morality is something God freely chose to create, and could have chosen to create differently, as opposed to something inherent in God's inalterable necessary nature – is one many in the Christian tradition reject. Certainly it is traditionally rejected by Catholics and Orthodox, although Protestants are more mixed in their views. Among Muslims, that idea is accepted by the Asharite theological school, arguably the most influential in Sunnism, but rejected by the other major orthodox Sunni schools (Maturidi and Athari) and also by most Shi'a theologians.
> If you purport to not believe in some sort of religion (loosely defined), how can your metaphysics allow for social norms to be judged by anything other than some sort of empirical fitness function?
How loosely do we define "religion"? I think a person could accept King's doctrine without accepting King's theism, but maybe any such position is in some sense a form of "non-theistic religion"? It isn't clear how to answer that question.
A person (even a non-theist) can believe in the metaphysical existence of objective ethics, yet reject any attempt to reduce ethics to "some sort of empirical fitness function"
Sure. Often, though not always, norms change because of technological or economic change. Lots of taboos on sexual behavior, for example, that changed in the 20th and 21st century arise out of the requirements of subsistence agriculture societies (such as the need to have many children to work farms and survive). Those changes aren’t arguments against norm enforcement.
Does that mean witch-burning was a good decision because it carried the weight of social consensus?