The definition of a slur is a derogatory or insulting term applied to particular group of people. The word ginger has been used an insult against only people of red hair. It is by definition a slur, but people do not recognize it as offensive. Please help. Also, red heads are predisposed to look younger and face workplace discrimination for that. 

See article here 

https://www.chicagotribune.com/lifestyles/sc-fam-0110-redheads-20120110-story.html