It seems like you are making several arguments, most of which dont really hold weight, one of which probably does (but I would need to see stats). First, just because a slur has been common usage for a certain population for a long time this doesnt make it less of a slur. No matter what the population or the slur. Second, as many people have said in the comments, saying that only white disabled people have issue with the term erases black and brown disabled people. Third, the disability movement is relatively new, so saying that people werent called out for ablist language over 20 years ago discounts the fact that our awareness of ableism AND out examining of offensive language is evolving rapidly and recently. Fourth, Lizzo seems to me to be a stellar and empowered individual, and perfectly capable of reacting defensively herself, if she felt criticism was unwarranted. Instead she did a beautiful thing by simply acknowledging, apologizing, and making the change because she doesnt want to contribute to ableism. She didnt seem to feel the criticism to racist, otherwise I imagine she would feel empowered to say so?

But to your fifth point, it may be true that Black artists are called out more frequently for offending. I dont know the stats on that but I do know racism is rampant.