Blaming 800 years to only Imu wouldn't be accurate.
Imu or not, Celestials would act same. Holy Knights would act same, they would still use slaves.
You think Imu is personally ordering them to buy more slaves?
We literally see Celestials cheering for Garling when he kills a good Celestial, they are not doing it based on orders, slavery is their part of their culture now and that they genuinely want slavery and they want hunting contest on other human beings.
Imu is the guy who takes the political decisions and appoints 5 Elders but Celestials under them would still make people slaves for 800 years without Imu as well.
Oda is applying the medieval era to One Piece, medieval era is where slavery existed in different continents even in different cultures, all different countries at some point used slavery system, if you think slavery wouldn't exist in One Piece world if Imu wasn't the king you are too naive.