You don't seem to understand 'What is Morality and Ethics".
Morality and Ethics involved Universal Principles applicable to ALL humans.
As an analogy from Biology, it is a Universal Principle in Biology and human nature, that ALL humans ought to breathe without exception.
This oughtness to breathe do not need enforcement but is spontaneous else the person will suffer pains.
There are exceptions where 'some' humans may choose to commit suicide via asphyxiation, but such minor exceptions do not deny the existence of the Universal Principle of Human Nature, that "ALL humans ought to breathe."
Similarly, Morality and Ethics involved Universal Principles applicable to ALL humans.
As such, since Morality involves Universals and Slavery is one of the Moral Elements,
so, 'No humans ought to enslave another' is the Universal Principle applicable to ALL Humans,
otherwise, the universal principle would be "Humans ought to enslave another" which is abhorrent!
There will be humans who want to enslave other humans and some humans want to be enslave by others, and these has to be taken as exceptions to the Universal Principle and must be investigated to why they are the exceptions and therefrom be mitigated.
Since 'No humans ought to enslave another' is the Universal Principle applicable to ALL Humans, it is objective, i.e. independent of any individual's belief, opinions and judgment.
To be objective and as a fact, the above universal principle must be empirically verifiable and justifiable with reinforcement by philosophical arguments.
In addition, this objective moral fact must not be enforced nor imposed on any individual as EXTERNAL authoritative Laws, rules, commands, and the like.
They are merely to be used as a Guide for the individual to improve his moral competence.
The above is applicable to all moral elements to be recognized as moral facts, thus objective moral facts.