Different countries use different formal terms for this.
In the US, “r” was the formal, respectful, preferred, and correct term for decades. E.g, ARC, a major organization helping people with this disability, had “R” meaning what you think it means. But it was nevertheless used as an insult, so things eventually changed and the nice word became unnice.
There’s a weird process where the formal term becomes delegitimized. It happens with ethnicities too. Sometimes old terms come back around too.
I’m not even sure what the preferred US term for “r” is now. I use “MR”, because a relative who’s married to a special educator uses it. I would guess that’s a medical/special ed term, but someone also thinks it’s wrong because they prefer some newer neologism.
I think that linguistic pattern is also about establishing authority, in groups, and boundaries. Ever changing cant makes it easy to exclude and demean others who didn’t get the memo of the term change.