That's a good question, which is a very difficult one to answer.
I think it depends on the context and the audience of your novel.
In my case my novel is mainly military, so it is a given that there will be insults. But even so, they are used very sporadically, mainly to reflect the desperation of the protagonist, who is a person with good manners and relaxed, but when things get bad he becomes much more aggressive and authoritarian, you must when he insults his allies to for example reflect that he wants one of his orders to be carried out quickly and directly.
No one has had any problems with the vocabulary.
But then even then some curious things happened with some "special" cases of readers.
for example, my novel is gender reversed, the protagonist is trapped against his will in the body of a girl, but is still identified as a man, well apparently someone got upset when he called an enemy soldier a faggot during combat, in a situation where both were trying to kill each other at any cost.
And later, when the protagonist has an episode of reflection, where he is discussing the situation he is in and he calls himself a faggot as a sign of contempt, everything was fine.
So I don't know what to think anymore.