TempestTost If cis was means to be inherently dehumanizing or diminishing, that would be a different story. That's what a slur is.
'Slur' has a wider use these days, it refers to words that are deemed unacceptable or offensive, to a person or group.
I don't believe there has to be malicious intent behind it, or that it has to be 'inherently dehumanizing or diminishing', I'm sure there are lots of cases where people have said something or used a term for a group which is not inherently dehumanising or diminishing, but is not, or is no longer, acceptable.
It is the settled opinion of the group concerned that determines whether it is offensive or not. It's not acceptable for Group A to come up with a label for Group B, and tell them they can like it or lump it.
So it's not acceptable for Group A to tell Group B that they shall be called 'cis', even though they don't like it, don't agree with it, contest its meaning and find it offensive. Therefore, I think the current use of 'slur' is valid here, as 'cis' it is deemed unacceptable and offensive to the people to whom it is applied.
It's no different than a transman or transwoman trying to stop you for referring to them as their sex.
Sex is an observable biological fact, and trying to stop people from stating observable biological facts is a different thing altogether.