Slavery is literal as well as an allegory, as slavery can be seen as an allegory for sin.
Also, the Biblical slavery you speak of is akin to how the Bible talks about divorce.
Matthew 19:3 Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife for any and every reason?”
4 “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female, 5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh. So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
7 “Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?”
8 Jesus replied, “Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. 9 I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery.”
God's plan for mankind was never for slavery just like his plan was never for divorce, rather, slavery was simply a consequence of fallen man's sin. It is true that Mosaic law permitted slavery, but with exceptions. Keep in mind that the entire world during that time was completely immersed in slavery. In fact, if it were not for the Biblical text, we very might all still be slaves. But Moses was sent by God to free the slaves in Egypt who toiled 24/7 to the whim of a dictator. For the first time, the slaves were rebelling and leaving in mass. Once they were on their own, there were people within the tribes who struggled to make ends meet, so they were allowed to be owned for a period of not more than 7 years, after which time, they had to be set free again to try and make it on their own again. This provision allowed people to survive in a fallen world, much like the Biblical allowance for divorce when the other party refuses to treat the other properly. Also, for the first time in history slaves were given a day of rest during the Sabbath which was unheard of. This provision is why we even have a weekend of rest from our work. So, no, God did not completely rid mankind of all slavery because God did not completely rid mankind of all sin yet, but it was the beginning of doing so. At some point, it will all be gone forever.
Compared to the slavery of Black in the 1800's, however, this was nothing comparable to Biblical slavery. That slavery was the abuse of an entire race of people to accumulate vast amounts of wealth indefinitely, and all based on race.
