At least here in the US, it's generally no longer considered appropriate.

https://www.specialolympics.org/stories/impact/why-the-r-wor...

The word itself isn’t wrong per se, but it has a negative connotation that’s associated with “slow” people (from the medical terminology “mental retardation”). It’s a playground insult to call someone “retarded” because they’re “stupid”. It could just be an American thing, but using it like that is very common.

I think a good comparison is “fag”. Sure, it means a cigarette, but it is (was?) a common insult towards gay people.

Should we not use the word? I don’t know. I’m just explaining why dpeck wrote their comment.

It's perfectly fine in a technical context: "bacterial growth is retarded by refrigeration" or something of that nature.

But that's not how the commenter used the word. There are other, far less ambiguous and far more polite words to refer to a general lack of progress... old-fashioned, primitive, backwards, antediluvian, obstinate, ...

It's not wrong. Imo the issue isn't the treadmill - that's natural, and will always happen with or without PC police. Unacceptable words existed long before PC police (in the modern sense) existed. Communities have long self moderated acceptable words.

The real debate is about how fast the treadmill moves.