It does actually mention it's an insult a lot of the time half way down the page. You wouldn't use it as an ethnic slur like 'roast beef', people accuse others of being a gypsy or a pikey. It is 'considered to carry pejorative connotations' as wikipedia delightfully puts it.
The word 'slur' is not mentioned once on that page. At best, its use as an offensive slur is context sensitive (at least according to http://en.wikipedia.org/wiki/List_of_ethnic_slurs).