The whole bible is about liberation from slavery (of sin) and quite clear about being against slavery (physical).
Read of Israel's salvation from Egyptian slavery? Joseph? Limits on slavery (7 year max) because people were too stubborn to accept not having slaves. The year of Jubilee?
The bible has multiple verses codifying how to treat slaves. All sorts of rules and protocols. The old testament does not say "slavery is a sin" or "slavery is forbidden". It clearly says "slavery should be done this way". An as soon as you do that, slavery becomes accepted.
It has provisions to help limit the harms of slavery, this is not the same as actively promoting it. It's similar to the allowance of divorce (and polygamy) in the OT which Jesus says was simply allowed/tolerated due to the hardness of their hearts.
There’s plenty of bits about fighting the neighbouring lands, killing all the men and taking the women for “marriage” (which, let’s face it, is a fancy way of saying sexual slavery and rape).
The Old Testament is full of these battles of conquest...
Read of Israel's salvation from Egyptian slavery? Joseph? Limits on slavery (7 year max) because people were too stubborn to accept not having slaves. The year of Jubilee?