I.e. "True slavery means that police/authorities won't intervene". It shows that you don't have a well-defined definition of slavery, or are using one that is too-constrained to the generally-accepted definition of slave/slavery. Here are a few:
[...]a person who is the legal property of another and is forced to obey them.
[...]a person who works very hard without proper remuneration or appreciation.
[...]a person who is excessively dependent upon or controlled by something.
You're citing the oxford dictionary. If you look at the example sentences, the latter two cases are figurative, not literal. Only the first definition refers to literal slavery.
However, I agree there's ambiguity. One of the additional example sentences of the first definition includes a case where the slavery was illegal.
If someone is held as a slave and effectively beyond the reach and help of the law, then it does them little good to know that their situation is technically not tolerated by society.
But citing the latter two definitions of slavery from Oxford is not a good way of making this point. Those definition are incorrect, when referring to literal slavery.
Probably what most people mean when they say slave is something like "a person who is the legal property of another and is forced to obey them, or a person who is illegally held as such yet beyond the recourse of the law"
[Click on "more examples sentences" to see the broader meaning of the definitions cited]
But let's be clear, you've essentially used this fallacy (not to mention the appeal to emotion) to support/base your point: https://en.wikipedia.org/wiki/No_true_Scotsman
I.e. "True slavery means that police/authorities won't intervene". It shows that you don't have a well-defined definition of slavery, or are using one that is too-constrained to the generally-accepted definition of slave/slavery. Here are a few:
[...]a person who is the legal property of another and is forced to obey them.
[...]a person who works very hard without proper remuneration or appreciation.
[...]a person who is excessively dependent upon or controlled by something.