This resource is dropping woke bombs left and right (I'm into it). Extremely rich text.
... societal interpretations of and responses to specific differences from the normed body are what signify a dis/ability.
Not to glom onto this one pat definition, but it's one of the many paragraphs that jumped out at me.
I still haven't got to any part that is specifically challenging using "polite" language when speaking to disabled people about their disabilities. But with your question about why I might consider the term disabled a slur, and these links, I think I'm getting there...
EDIT: uhhhh nvm, the paragraph literally after the one I quoted goes into it, very specifically.
Some advocate for People-first language (a person living with disability), while many disability activists advocate for identity-first language (a disabled person).