Exactly. There’s even an opposite of a euphemism, “cacophemism,” albeit, you don’t hear it much.
Examples would be calling sexual intercourse either “making love” (euphemism), or “doing the nasty” (cacophemism).
Those don’t shift much over time. It’s true that some become dated; hardly anyone uses the term “hanky-panky” for sexual intercourse anymore (or for anything-- it used to strictly mean sex, then it went through a phase of meaning just doing something you shouldn’t be doing, then it pretty much faded from use altogether). It could come back into vogue as a euphemism if it were to be used in a popular film set in an earlier time period, or something, but it will never become a cacophemism. Words don’t work like that. Euphemisms blur some sort of reality you don’t want to state directly-- “going to the bathroom”; “passing away”; “in his cups”; “in the family way.”
Cacophemisms take the messiest part of whatever you are talking about, even if it’s a small part, and brings it to the fore. “Knocked up,” makes the “had sex” part of being pregnant the most relevant part-- not that there’s anything wrong with that, but up until the actual birth, it’s often the messiest part. Depends on how much morning sickness you have. But at any rate, “knocked up” will never migrate to being a euphemism.
My point is, that the terms the OP cited, terms for racial identities, DO migrate. Today’s most PC term is tomorrow’s insult. “Colored people” was the prefered term once upon a time. Not in my lifetime, but in my grandmother’s, a woman who always strove to be kind and call people what they wanted, but on one rare occasion did slip up and say “colored people” in front of my then-19-yr-old brother c. 1991. Frere Crusader lit into her, at which point my mother told him the story of growing up in a mixed neighborhood in the 1940s, and how all the children played together, but there were still some unspoken rules about going into certain houses, and she was the only little white girl who bought presents, dressed in her nicest party dress, and attended all the birthday parties of the black girls on her street. My brother was about 6 inches tall after that.
But he was right that in 1991, “colored people” was not a nice term.
However, terms that migrate are neither euphemisms not cacophemisms. Those have immutable qualities, so they don’t migrate.
What the OP referred to are simply slang terms.
They come and go, because people are cruel, and as soon as a new, empowering term comes to the fore, there will be people who use it in such a way as to seek to disempower it. This is why some groups fight back by reclaiming terms that were once insulting (cf., “queer,” and “dyke”): “If I call me this, it won’t have any effect for you to call me this.” So a term is in, then out, then in again. But that has nothing to do with the realm of euphemism.
Well, a little, I suppose it does, since the OP did touch on the use of euphemisms for some terms for disabilities.
I’ve worked with and around disabled people my whole adult life, and on the whole, they hate circumlocutions to refer to their conditions. I was an American Sign Language/English interpreter for 15 years (carpal tunnel syndrome forced retirement), and to a person, Deaf people who are members of the Deaf community reject the term “hearing impaired.” The only people I have ever met who used it were late-deafened adults, and even many of them reject it, albeit, some prefer “hard-of-hearing.”
People who have physical disabilities generally hate alphabet soup terms like “physically challenged,” or “differently abled.” Most people prefer to be referred to by what they actually have-- CP, MS, MD, but barring that, “wheelchair user” or “amputee” or whatever is easily observable, is preferred over “physically challenged.”
I’m not sure why some people have felt it necessary to come up with long-winded terms, so I can’t really say whether the intent was meant to be euphemistic, or just inclusive-- “physically challenged” could mean anything, from an upper limb amputee to someone with CP to a lower body paraplegic. We already had “disabled” for that, but since terms tend not to migrate, I think it’s less of a euphemism, than someone trying to hurry an older term off the stage to usher in a new for, though to what purpose, I’m not sure.
OK. Need to wrap this up. Never intended to ramble on. English major hazard.