True, though “bitchin’” isn’t used as a slur in this context, it’s slang for “cool.” Since like 1970 too, it isn’t some “new rizz.”
If we want to get technical, a slur is “A disparaging remark; an aspersion.” So by the dictionary, any negative remark, especially one that is a false or damaging accusation or insinuation, is “a slur.” Therefore “you are a silly goose” counts as a slur, as it can be contextually negative to be seen as silly, and it is false to insinuate that you are a water fowl.
We don’t really treat the word “slur” in real life as it’s definition would suggest, or to reign in my above joke a bit, words like “fuckface” or “dick” would commonly be considered a slur. Instead they’re just considered negative remarks by the community at large, to be a “slur” in the eyes of the public (not the dictionary) it has to have something more than just “bad.”
I posit that “bitch” is truly no different than those common “bad” words, and should not be considered a slur. Frankly, same for “cunt.”