Writing an effective slur filter is undesirable. No one can stop people saying completely shit headed things. For one, people just work around it. And more importantly for every shit headed thing you restrict you also cut out someone else, even if it's just because of their perception of constraints. The mind has a funny way of editing thought before it even arrives. We can't afford this anymore in our online spaces.

There's also a language problem: "retard" is a common word in french which bears no insult in it and simply mean "late" like in "being late to work".