>>1509924The only time I can remember seeing an isekai protagonist actively fighting against slavery is in the manga Grand Dwarf ~ The Smalltown Craftsman's Skills Are Unmatched In Another World ~ though he seemed more upset regarding the treatment of woman and children, rather than slavery as a whole.
Reincarnated as a Sword is also anti-slavery, but that's less to do with the MC's personal views, and more the fact that he's wielded by a former slave.
