#this is probably why we know to use google and wikipedia better than usamericans and even brits

[hussyknee]

The r-word is still a slur, and the fact that you don't know what cretinism means doesn't mean nobody else does. It was used in my school textbooks in South Asia, which are still influenced hugely by British colonialism. A lot of people didn't know "midget" was a slur either, to the point that John Mulaney used it repeatedly in his godawful stand up just because his producer told him not to use it. There's lots of slurs nobody had any idea were slurs, to the point that Beyonce apologizing for using the word "spaz" got immense pushback. Meanwhile, the first time I tweeted this, a mother of a girl with CHT thanked me because her daughter was triggered every time someone used it.

As for "where the line is" I pointed out clearly that slurs for specific conditions, historical or otherwise, should be off the table, rather than general ones that's lost their relevance like "idiot" and "moron". Even so, "imbecile" still toes the line because it was in wide use as the bottom-most rung of the intellectual disability hierarchy pronounced in 1927 by Henry Goddard. It was the last word to fall out of use with the introduction of "mental retardation" in the DSM-IV in 1994. Definitely not equivalent with the word "queer" which was actively reclaimed by the community during the AIDS crisis.

PSA: Please for the love of God stop using the word "cretin". It's a slur for people born with congenital hypothyroidism (CHT, that used to be called cretinism) that causes physical and intellectual disability.

I think it's unrealistic to expect a blanket moratorium on insults about intelligence, but words like "mong/mongoloid" (anti-Asian slur later applied to people with Down Syndrome), "spaz", "downie", "midget" and "cretin" refer to people born with specific developmental disorders. If you care enough not to use the "r–word", please steer clear of these as well.