The usage of the word "slur" in question -- it has to be this if it's on a list one can learn from DEI consultants -- is
a derogatory or insulting term applied to a particular group of people.
It is inherent in the term itself, not in its use.* So it isn't simply anything that can be understood as an insult. All the stuff about "punching up" and so forth is beside the point.
"cisgender" has a technical meaning which is still it's primary use: someone who identifies with the gender they were assigned at birth (so it can apply to intersex people as well). In this it is like "heterosexual" and "male". Arguably it is not like "white", in that who counts as white is malleable, but for the most part whatever it is, in most contexts, "white" is not a slur either.
* I am in fact a lapsed linguist. I have a PhD. My specialization was in semantics and pragmatics. Semantics is meaning encoded in language. Pragmatics is meaning inferred from use: "it's cold in here" meaning "shut the window", for example. I am aware that one can talk more precisely and at much, much greater length about all of this. But this is Hacker News, so this is all I am going to say.
"cisgender" has a technical meaning which is still it's primary use: someone who identifies with the gender they were assigned at birth (so it can apply to intersex people as well). In this it is like "heterosexual" and "male". Arguably it is not like "white", in that who counts as white is malleable, but for the most part whatever it is, in most contexts, "white" is not a slur either.
* I am in fact a lapsed linguist. I have a PhD. My specialization was in semantics and pragmatics. Semantics is meaning encoded in language. Pragmatics is meaning inferred from use: "it's cold in here" meaning "shut the window", for example. I am aware that one can talk more precisely and at much, much greater length about all of this. But this is Hacker News, so this is all I am going to say.