Retarded was invented to be a “kind word”. It’s humorous looking at records from Ellis island and seeing records describing people as “idiots” “imbecile”, and “morons” as actual terms to describe different levels of intelligence. We use these words outside that context now. But they were actual classifications. Words like “retard” came into being to cover those terms which became derisive slang that is today considered harmless. But retard isn’t.

https://www.merriam-webster.com/words-at-play/moron-idiot-im...

Obviously teasing an actual mentally retarded person by calling them a “retard” or the above terms is cruel and in poor taste, and worse. I don’t believe almost anyone would though.