In philosophy, some take the absolutist position that lying is always wrong, no matter what. Famous philosophers who have embraced that absolutist position include Aquinas and Kant.

Does anyone approach racial slurs with the same moral absolutism? I don't know. I know less about Kant, but Aquinas (and his followers up to today, for example Edward Feser) wouldn't limit their moral absolutism about speech to just lying, they also include blasphemous speech and pornographic speech in the category of "always wrong, no exceptions" speech. If one believes that lying, blasphemy and pornography are examples of "always wrong, no exceptions" speech, what's so implausible about including racial slurs in that category as well?