personally i do consider it a slur and avoid it accordingly, but i think the difference with this one is that colloquial usage took it over entirely. i'm fairly sure the average person does not think of down syndrome when they hear that slur, they just think of a particularly stupid person. which is also how the words "idiot" and "dumb" stopped being slurs over time, even though those also used to refer to medical conditions.

this might differ between cultural backgrounds, i suspect that's why it was categorised as a slur in contemporary internet culture. but it's pretty hard to find another slur that doesn't unambiguously, inseparably refer to a particular group it degrades. with the r-slur, some people do appear to separate it, i think that's why they insist it's okay to use.