Cis is not a slur but certain cis people consider it a slur because it is a way to categorize them that does not automatically mean ‘Normal’ or ‘The way one should be’- They are offended that they are not being presented as the ideal. This is the reason. Even if it is subconscious, this is the reason.
Sorry but this is untrue. The first time I even learned this was being applied as a label, it was said and explained in a derogatory manner.
Whatever impression I have about this particular label being applied to me regarding the absence of “normal” is a direct product of this experience.
CIS is negative, and only applies to computer science studies in my opinion.