In those days, people with mental illness were often stigmatized by society.

stigmatized 是动词 stigmatize 的过去分词形式，在句中表示被动。stigmatize 由名词 stigma（耻辱的标记）加上动词后缀 -ize 构成，意思是"打上耻辱标记、污名化"。stigmatized 表示"被污名化的、被歧视的"。