"A slave is property."
Yes, and so are citizens of a state.
"Inherent to slavery is some form of dehumanization."
No. The definition of a slave (first hit on google) is "a person who is the legal property of another and is forced to obey them." In ancient times, people who lost wars may have been subjugated as slaves, but there was no pretense that they were less human. I think you might be thinking of racial slavery, which was inspired by a belief that other races were subhumans.
"being somehow below the ruling class"
Even ordinary citizens, who may be able to vote and do all that fun civic stuff, may still be below the ruling class. As Lysander Spooner said, "A man is none the less a slave because he is allowed to choose a new master once in a term of years."
> "but you have remedies, such as renouncing your citizenship"
But for US expatriation, you will have to pay an exit tax as well as continue to pay income taxes, so no you aren't able to be free of bondage. Getting to pick from a new master on a different tax farm doesn't mean you are not a slave.
> "leaving for an area not governed by a state"
If there were such a place, then pretty soon, some state will likely come in and claim you for their tax revenue. Considering that states make this threat means that your relationship with them is not voluntary. Also, the "you are free to leave" argument doesn't take into account the fact that you never voluntarily entered into a relationship with them to begin with. It doesn't mean you were not a slave while under bondage. Effective slave masters realized that by giving their subjects some freedoms, that they were better off. But just because you have some freedoms again doesn't mean that you are not a slave. You are still the legal property of the state and forced to obey them in matters which the state desires you to obey.