This was the top post when I checked Lemmy this morning.. Mod commented to mark the post NSFW, even though it wasn't NSFW as far as I could tell. Post was locked and had a ton of deleted comments.
The issue is that "retarded" is considered an ableist slur though, like the n-word is a racist slur.
Tap for overthinking tangential thoughts
While the Merriam Webster lists slur and insults as synonyms, I observe slur to be used describing insults based on a terminology degrading other people than the target of the insult by using a derogatory term towards these people as an insult. So retard would be a slur (and an insult) while asshole would be an insult without being a slur.