In the past, people with mental illnesses were often stigmatised by society.

stigmatised 是动词 stigmatise 的过去分词形式，在句中表示被动。stigmatise 由名词 stigma（污名、耻辱标记）加上动词后缀 -ise 构成，意思是"给...打上耻辱标记、污名化"。stigmatised 表示"被污名化的"。