That:
"In everything, do to others as you would have them do to you"
outlaws
any kind of mistreatment of others (not just some specific wrongs, such as the lists of laws in the Old Testament), including every last instance of any kind of poor treatment of
anyone.
Such as even being merely harsh and unfair (even just that) to an indentured servant, or a circa 2018 employee, is now outlawed for Christians.
But much earlier than this, in Israel, already, way back in the OT time we talked about, micro regulation laws
from God regulated slavery:
15 If a slave has taken refuge with you, do not hand them over to their master. 16 Let them live among you wherever they like and in whatever town they choose. Do not oppress them.
-- Deuteronomy chapter 23.
Yup, that means precisely what it sounds like it means.
That
law from God (no less) says any escaping slave/servant, say from a village a few miles away for example, is to be treated as a fresh new citizen, and treated well (not merely tolerated).
So...it's instantly impractical to mistreat your slave then, for anyone obeying law from God.
Can you see that?
But in the New Testament, later in time, Christ gives us the perfected form of the golden rule, which says we are to treat everyone, everywhere, in all circumstances, all the time, as we would want others to treat us if we were in that situation they are in....
Which not only expands and extends the Deuteronomy 23 law on escaping slaves, but goes much further.
Like Paul's short, 1-page letter
Philemon to a Christian about their own escaped servant -- that the escaped slave is not merely to be treated ok in their new location...
but is now the
social equal of the previous owner, fully. That's revolutionary of course.
It would be amazing if today in the United States we lived up to that level of treating
anyone as a true social equal....
Of course only some people obey the golden rule from Christ (in it's total form), when it's harder to do. They have help though, from an invisible source (to use the wording of Joseph Campbell).