@Wasanelder,
Jewish males could be freed. Females were not (if I recall correctly).
Also the female slaves could be 'married' to their owner or his son just like that (I call that rape and sex slavery).
And foreign slave (those captured during wars) were never freed.
Regardless of all that, those were Jewish customs. The letter to Timothy is from the Christian era. They had to deal with Jewish, Roman and Greek slave owners, all with their own customs regarding slavery. Some (classes of) slave were treated relatively well, others were brutally abused.
How hard could it be for God to tell Christian slave owners to free all their slaves?
How hard could it be for God to tell people it is plain wrong to buy, own and sell other people like cattle, and deny them basic human rights and freedom?