I didn't think cishet was a slur. I thought it was just an internet thing, since we shorten/abbreviate everything on the internet.
I used to think cisgender was an insult because of the way transgender people used it. But I think part of my aversion to the term "cis" was knowing that it didn't describe me without yet realizing why, lol.
I think if more people used these terms in everyday language, rather than using them to express bitterness, they would become more accepted by the general public. No cis person identifies as "cis", though. It's just a term we use because it's easier than saying "non-trans non-intersex". People don't like being labeled by other people, even if there is no bitterness attached.