20% was the high end too. Lots of citizens of ancient Egypt paid 10-20% of their yield and they were lenient depending on if there was a bad harvest or not. Slaves were property acquired through conquest and other socioeconomic conditions but there were a fair amount of debt slaves there and throughout other ancient civilizations. 

It feels like we are in a system where the majority of people are debt slaves and they don’t know it. Debt slaves plus taxed far higher than ancient Egyptians.