From Wikipedia, while the concept has been discussed since at least 1914 the actual term cisgender has apparently only been used since 1994: https://en.wikipedia.org/wiki/Cisgender
Cisgender is recent, but it’s also primarily used to serve as the counter to transgender. To understand “cisgender”, it’s useful to study the history of “transgender”. Transgender was introduced as a term because it encompassed a larger group of people and avoided the stigma surrounding “transsexual”. Transsexual has a history dating back about a hundred years. Since “trans-“ and “cis-“ are antonym prefixes, once you have transsexual, you have cissexual. For the purposes of Elon’s rule, it doesn’t matter whether it’s CISsexual or CISgender, he objects to “cis”.
One reason why many people, particularly feminist women, find "cisgender" a disagreeable term is because it's a linguistic concession that makes it more difficult to argue for the rights of actual female women, as it implicitly redefines "woman" to include males and "man" to include females.
Disagreeing with a particular taxonomic way of describing different types of people simply because it makes it hard to advance your political beliefs does not mean the words involved in that taxonomy are slurs. “Slur” has a particular meaning, and it isn’t “makes it hard for me to achieve my political goals”.