I heard that in the Yuan Dynasty, Han people had to take off their thumbs as soon as they were born.

I haven't heard of it. Not feasible. No thumb is no different from no hand. Four fingers are like shovels, and nothing can be done. How can you serve the Mongols? However, in the Yuan Dynasty, people had the habit of throwing their first child to death. Because the Han people get married, the Mongols have the right to have their first night, and the newly-married Han brides have to serve the local Mongolian lords first on their wedding night.